This post is my personal take on the state of the game, on what it needs to be changed or fixed. I do not claim that I have not misunderstood single points nor that the list is complete, nor that what is fun for me is fun for everyone (especially those who love the RNG game Diablo 3 and how things were there).
It is a very long text as it is mostly meant as descriptive feedback for Blizzard themselves, so please do with it what you consider best.
GOOD
A) Story is great, a sheer 9 out of 10. The only two points I see for missing the 10 out of 10 rating are that …
A.a) it is again a young girl who gets tortured by a greater evil. While of course I do not oppose girls/women to be protagonists in stories, especially if they are made as good as Leah in Diablo 3 or Neyrelle in Diablo 4, it does not need to be a young girl every time that is the carrier of suffering. This translates as a bit of a try to artificially increase the felt burden of the greater evil. Early Diablo games had the adult male adventurer/wanderer, it would be nice if the next candle gets carried by an adult woman instead of a chield 
A.b) One or another inconsistency in the new Vessel of Hatred chapter of the story.
!!! SPOILER !!! >>> E.g.
I) How does the Church of Light not know what happened, what the wanderer achieved?
The Church of Light seems to know everything that happened, what the wanderer and his/her team did and achieved, from the descent into the lair of Rathma so that Vigo can help us there to the final battle against Lilith before which the entire army of the Church was eradicated except for Urivar and Prava, none of whom were in any condition of following the wanderer to see what happened given how they survived. Who was the one who bragged to the church about what all happened so that the church could sentence the wanderer and his/her team as heretics so fast, establish a ransom on their heads and attack them to kill or incarcerate them? How does the single, severely wounded knight think that he could best the wanderer who singlehandedly and alone defeated Lilith, a task even his “god”, the Angel Inarius, did not manage (and which he knew!)?
II) Why do we not meet Lorath to search further for Neyrelle?
Aside of the base game campaign showing Neyrelle seems to travel a dire winter area from where she left the letter (Fractured Peaks) to - apparently - arrive in Gea Kul where she finally boards a mid sized ship and finally sails off over a huge body of water (likely the inleand ocean in the west of the playable map). That happens at the time when we still read her letter, and pursuing / trying to find her we just pass a small rivulet to the south, dividing Kehjistan and Nahantu, and basically find her pretty soon after.
Lorath is nowhere near. There should be some kind of explanation why he is not there, and why we do not inform him to come once we heard (in Gea Kul) where she likely went.
III) How can a low level grunt succeed to kill “the wanderer”? (or at least present a very good try at it)
We meet a faith of light extremist / grunt knight who accidentally (not by skill!) survived the hell war with heavy injuries, whom we do not trust from the start, and who could not hold our water in a fight against a dumb hellhound, a horde of those or, god beware, Lilith herself - Why do we turn our backs to him so he can backstab us with his oversized toothpick? Why does our armor - which protected us against the assaults of the highlevel lesser evils Duriel and Andariel, much as against Asteroth and Lillith, whom no human on Sanctuary could stand up against - not protect us against this level 15-25 grunt’s attack?
There should be a bit more strength and cunning involved to defeat “the wanderer” who did what even the angel Inarius could not do, which likely even the Arch Angels off heaven might have issues pulling off.
III 1/2) Speaking of the Arch Angels, why did they not care about what happened in Sanctuary? They could ignore Inarius (if he reached them with his plea to return to heaven at all), but did not mind in the first place that Lilith plotted to absorb Mephisto’s, a Greater Evil’s power to add to her own? Did Tyrael have so much trust in humans still that he though another Nephalem would surely show up, and did the hubris ridden Imperius finally conclude that he should trust in the help of humans sufficing for what he would consider his job until and including Diablo 3?
IV) Why do we not try to recover the soulstone after Eru brought it away?
We are always one step behind Lilith’s plan and need luck, cunning conclusions and the hubris of Elias to catch up repeatedly. >>> We defeat Asteroth to finally get the soul stone and put a lot of effort into preparing the stone. >>> We lose the soulstone to Inarius and the Church of Light and reclaim it in the middle of hell after 2 huge armies clashed and destroyed each other. >>> We fight through hell, the realm of Hatred itself to both defeat Lilith and imprison Mephisto. >>> >>> But when Eru hides Akarat’s body and the soulstone with Mephisto inside of it, we just shrug and give up? Neyrelle’s entire suffering turned in vain and she does not even care anymore because we told her that she has overcome her burden? An Angel taking the soulstone from the wanderer could not stop him/her to continue the fight, but an old, fragile human, a dodderer from the jungles can?
That is so unheroic, so unlikely for the wanderer and for Neyrelle.
V) The corpse of dead Akarat turns into a vessel for Mephisto by just laying the soulstone onto his chest? All other soul stones since they were first used had to be rammed into the skull of a living human to render that human into a vessel for the Greater Evil. If such a loose contact is sufficient to have a Greater Evil turn a human into a vessel, how comes that Mephisto did not just made Neyrelle his vessel in secret? How then are soulstones any kind of safe prison for the encapsulated evils? Or did Eru use the soulstone to smash dead Akarat’s breastbone to implement the stone sidewards into the body, knowing enough about soulstones to know that such could free the Greater Evil too? Or did Mephisto tell him that it would work?
This very end seems to have traded a bit of consistency and “inworld logic” so it could be rushed to finish.
<<< !!! SPOILER !!!
B) The concepts of the 6 single classes are great (excl. balance aspects for now).
C) Graphics are great. For all aRPGs I know it is the 10 out of 10 by large margin. There does not need to be said much more about it.
D) Open world area is wonderful. 10 out of 10. If any, it could be 10 times as big and I would love it 10 times as much and rate it to be a 100 out of 10.
E) Group content (Black Citadel) is a very nice start for group enforcing play and should be expanded in future.
F) I like how additional regions of Sanctuary arrive by DLC, and I hope that the entire western side (where e.g. Tristram is located) will offer enough space to actually fight Mephisto, then Baal, then Diablo within a total of 3-5 more DLCs. And if every DLC would also add another fun class, maybe minus the overpowering bugs that overtuned the Spiritborn now, D4 would have an exciting future. I would love to see a roadmap explaining that way actually!!
G) I Loooooooove the clan I am part of, and I would hope that more clan management tools will be introduced for our officers, and that there will be more group- and clan-oriented content added to the game in future expansions (expansions, not seasons, as I want that stuff to stay).
BAD
1) Globally
1.1) Lack of testing and quality assurance
1.1.1) About every season one or another exploit comes that allows item duping.
1.1.2) About every season one or another build becomes OP due to bugs in how skills intermingle.
1.1.3) With the patch to introduce the 2024 halloween event, the client began to crash for people. And even 4 bugfix patches did not solve the issue, only the 5th patch covered at least most reasons for crashes as it seemed to me personally at least.
==> At least the PTR was given to players, but players cannot and will not test everything, cannot do load performance tests and do not have the resources and time or try many skill/item combos to anticipate bug-based broken or else overpowered builds like Blizzard can with their knowledge of the design and coding. Blizzard needs to do test automatisation (e.g. to discover unintended intermingling of single skills) and more relyable testing to prevent crashes and other bugs (e.g. the horse riding animations not being compatible with the introduction of animated wings during a former season if you remember).
1.2) Quality of life features are far subpar to modern state of the art
1.2.1) Inventory and Stash sizes are still far too small, and apparently due to “coding limitations” - which cannot be valid as other aRPG competitor games have by far more inventory and stash space (LE, PoE, PoE2, …). Also a new inventory tab for socketables cannot hold each one copy of every rune and gem type. Similar, Infernal Compasses which are all equal except for the wave count and cannot be stacked. Stackable items at times have absolutely senseless stack-caps (e.g. 20 for tempering scrolls while potion stacks cap at 99, others at 50, etc.).
1.2.2) While there is a Wardrobe (with its own shortcomings, see below), there is no way to safe different builds for single characters incl. skill settings, paragon settings, gear, etc. - at least as of season 6, season 7 will eventually introduce that. Also the game limits the count of characters distributed over all single worlds (season / non-season, hardcore / softcore, ssf, etc.) to a meager 12 without or 14 with the VoH expansion in total.
1.3) The entire concept of restarting on level 1 every season is based on non-existing reasons
1.3.1) The reason for all players on a season realm having to restart at level 1 with each char they want to play for every new season are the leaderboards that highlight various competitively won achievements like being first on max level, first to best max Dungeon/Pit/… tier, or first to get achievement XYZ. These leaderboards are not in the game, respectively not active anymore, meaning the reason for having to restart with blank characters is void and just another petty try to keep players longer in the game without offering the appropriate extend of contend for casual and for power gaming players to be busy for as much as possible of a 3 months season.
1.3.2) The vanishing of season mechanics at the end of a season is not state of the art anymore. Competitor aRPG games move those mechanics to the eternal/legacy world as well, either at the end of the specific season or partly even already at the start of the season. Hence the sole benefit of the season realm (and it’s enforced restart at level 1 each new season) is the leaderboard competition. And most of the season mechanics stay in the game then. Meanwhile players who are not interested in the competition are still able to use the standard season pass / buy the premium season pass and enjoy the new season content, work the season pass/quest/journey, etc. on the eternal/legacy world as well.
1.4) Mercenaries cannot be individualized (skills & looks) nor enhanced by equipping them with gear as it was possible in prior games of the series (D2 & D3). This is another (yet?) missed opportunity to give players content to farm, by that improve their characters’ power and hence overcome bigger challenges in game.
2) Reintroduced bugs & removed features
In Season 6 several already introduced aspects of quality of life were removed again:
2.1) Useless “Damage on Tuesday” affixes begin to return to the game: E.g. “Life per 5 seconds”, “+% Healing Received”, “Lucky Hit: Chance to Heal” and similar are not inline with the game’s implementation of other mechanics and game flow. They do not scale with the mechanics they shall influence, e.g. height of health pools, availability of health restoring mechanics per class or damage dealing style (few high dmg attacks vs. many low-mid dmg attacks), etc.
2.2) Backtracking in dungeons has returned after being reduced a lot for prior seasons
2.3) Loot drops are wasteful and frustrating again
2.3.1) Magic (blue) and Rare (yellow) loot items can still / again be found in Torment 4, high tier pits and other high level content. E.g. side quests, done in Torment 4 exclusively return Rare (yellow) items. These items will never be worn, will be looted only for mats or cash and clutter the inventory due to no way of having those items either automatically converted into gold or into mats. Similar: useless legendary non-ancestral items (i.e. without GA) with aspects that are no upgrade anymore - it should be possible to set these to be automatically converted into gold or mats too.
2.3.2) There is no mark in the game after which only max item power legendary gear (orange/ancestral) drops anymore, not to mention guaranteed ancestral items with 2 or 3 GAs. Instead the drop rate for ancestral legendary items is so abbysmally bad that even with spending 5.000 obols (2 full purses of obols!) AND about 5.000 Pale Marks (Merc Bartering), chances of not getting a single Ancestral item, much less an Ancestral item with required affixes / greater affixes are extremely high even on Torment 4. Also, no matter what ones does, from Citadel over Pits, NM Dungeons, Open World events, Undercity with or without Tributes… rewards are always the same useless non-ancestral junk. There is No guaranteed max item quality drops anywhere anymore as we had before. There is no guaranteed 2 or 3 GA drops in e.g. higher ranks pits. Instead item power 750 UNIQUE items partly drop more frequently than item power 800 / ancestral legendary items.
2.3.3) Linked to the unacceptably low drop rate of ancestral legendary items is the hence utterly low chance to find high to max roll aspects
2.4) The enormous grind for XP for level 1 through 100 in the early seasons, countered by many sources for XP boni and easy powerleveling ways (incl. Glyph leveling btw!), was brought back by an even more immense grind required to get from level 1 to level 60 + paragon 1 to 300. While before, and after Season 1, a player with medium level time invest needed maybe 1 week of “relaxed” playing to get a character to 100, now it takes 2 days of “relaxed” playing for level 1 to 60, 15 minutes if powerleveled, but many weeks of constant, mind numbing grinding in Pits to get to Paragon 300. The total time it priory needed to get level 1-100 for 6 characters was a walk in the park compared to the grind to paragon 300 now, even given that paragon are world wide (season / non-season, hardcore / softcore, ssf, etc.).
Here I will also include the massive lack of content of the D4 endgame. This entire XP grind is basically only done efficiently in the mid ranks of pits (85-110), respectively in Pit 150 for those able to speedrun those in under 2 or max 3 minutes. All other “endgame activities” were reduced to midgame events / Pit tier 65 / World tier Tormend 4 equivalents.
2.5) Riding is a bad experience again like in the early seasons of the game: The horse is getting stuck on every second corner in the open world, and it takes the slightest bit of damage for the character to be tossed off the horse and having to fight a horde of borderline-no-reward mobs before being able to ride on towards where one plans to be. Also it feels as if the recharging time of the horse compared to the gallop time is even longer now, so one rides more in the base trott speed, reducing the agro breaking of mobs significantly.
2.6) Even if not interesting for myself, the trials and leaderboards are switched off again, which are the SOLE reason for the new mechanics being restricted to the Season realms and hence also the enforcing of starting over the characters at level 1 every season.
2.7) The item to enhance the levels of mobs in the Hell Tide was taken out of the game again despite being so enjoyed by the players that it was kept for seasons 4 AND 5. Paired with the grotesquely bad drop rates of Ancestral legendary items, and the availability of the Boss mats drop rate enhancing Undercity and Opals, this renders the hell tide pretty useless.
2.8) World boss timer warning messages are broken again
3) Loot & Random Number Generator (RNG)
3.1) With the enormously low drop rate of max item power (legendary/orange/ancestral) gear, the core aspect of the aRPG video game genre - item/gear progression and the endorphins rush when getting an upgrade as base of the fun to play - is clearly a missed mark. This kind of drop rate, based on a lazily designed gear progression and world difficulty balance, leads to partly no gear upgrade for several weeks. And this comes from the perspective of a certainly above average power gaming (~6h/day avg) player.
3.2) The excessively high use of RNG in the following game mechanics leads to lack of gear progression and frustration due to the very high risk of the RNG results rendering invested farming time and effords useless, and repeatedly so. Furthermore there is no way to influence or mend/repair/reset the result of the RNG roll (e.g. by farming certain “luck tokens” to assure 100% the outcome of tempering, mastersmithing or the GA/GAs choice for mythic-smithing), which is the reason it can render days or even weeks of farming useless, which is extremely frustrating for players experiencing that.
3.2.1) Tempering has RNG not only for which effect out of the single recipes is added but also has RNG for the value of the added effect. Tempering meanwhile has a way to add a total of 5-6 rolls per item, but that still allows items to be bricked and does not allow to choose the wanted affix nor the height of the roll for it.
3.2.2) Master Smithing has no way for the player to influence which affix of an item gets mastered, leading to often millions or billions of gold and hundreds of thousands of obducites being wasted (both of which needs to be farmed with a lot of time and efford beforehand!). The huge amount of money can best be obtained by either farming bosses for worthless uniques (see next point) or playing the out-of-game diablo.trade game where one tries to sell single ancestral legendaries for billions of money in a totally destroyed and duped item / money flooded market.
3.2.3) Non-mythic unique items use RNG not only for the affix values but also for the aspect value, leading in often entire inventory filling loads of uniques being sold or scrapped due to being useless or too badly rolled to be used. For me, those boss runs are actually the main source for gold: 400 mio gold per evening, with usually not a single unique found that I could use as upgrade for any of my 8 characters and their specific builds. Pure frustration.
3.2.4) Crafting of mythic uniques uses RNG to determine onto which affix one or more GAs are placed
4) Class Balance
4.1) Class balance is an utter catastrophy, there is no better way to call it sadly. Each season an absolute of 1-3 builds amongst all meanwhile 6 classes and their countless build options are utter meta and the mightiest way to go by often huge margin while all other builds of all classes struggle and cannot perform nearly as good. Often that is due to bugs, exploits or apparent lack of knowledge of the game designers about which damage buckets can be combined how efficiently for a build. And after each season those builds usually get nerfed to uselessness, just so that another build of another class is in the same exploit-/bug-based overpowered position in the next season.
4.2) Following build guides or being able & having the time to create builds of similar power is an absolute must if wanting to partake in defeating high level pits. Beating these higher level Pits is required not only for the competition aspect of the game (which is the sole justification for the ever restarting on season realms) but also in order to be able to level up the character’s glyphs. With the current catastrophic state of class balance and the utmost abysmal drop rates for ancestral gear paired with skyhigh demands in gold and obducites to mastersmith items, almost outright required to beat tier 90+ pits, non-spiritborn classes have extremely unfair disadvantages in the glyph leveling process related to time and efford needed to get the glyphs to 100.
5) Battle System
5.1) The battle system is monotonous and boring - especially paired with the given battle events: trashmob floods with elites, boss battles. There is no need for diverse combat styles, e.g. single target expert, trashmob flood (“adds”) expert, Damage over Time expert, Debuffer, Tankiness expert, Recovery/healing expert (self healing in that), etc… Every class is laid out by it’s skills, if needed by aspects transforming a skill from single target to AoE, to deal with “the-more-mobs-the-better”, outright performs better if there are more mobs. Most Primary Resource recovery is based on and more efficient if more mobs are present. Every class has ways to become basically untouchable with shields, barriers, armor, resistances, etc…
My personal assumption is that this is not for the reason of making players feel more powerful, but to lessen the need of creating and actually planning/designing enough and more content to make each perspective of expertise (see second sentence of this point) relevant here and/or there.
5.2) The challenges for players does not come from the single types of encounters anymore. Either they can best an encounter or not - either they can one-shot a challenge (trash flood or boss) or they get one-shot. The development from not able to one-shot is a bit bigger for what players can achieve against mobs than what mobs can achieve against player characters. I.e. players have to work a bit longer from needing 10 Minutes to kill a boss to needing 1 Second to kill a boss – the time a boss needs to one-shot a player char over wearing it down to being unable to harm the player char is significantly shorter. It does not matter if we talk boss fights or trash mob floods with or without elites.
5.3) Ground effects like explosions, poison pools, shadow damage areas, lightning strikes and freeze clouds seem to not care nearly as much about player resistances than mobs doing comparable damage types does.
In the so called End Game nothing except those floor effects can actually harm a fighting player character anymore - and the key word is, as also indicated in 5.1, “fighting”: The more you fight, the more you achieve. If/while you fight you get tankier, if/while you fight you do more damage, the more adds a boss has the faster the boss dies (the boss events do not get more difficult, but more easy by adds). Ground effects target player character during battle and between battles, i.e. when their shields, barriers, etc. are down, and hence usually one-shot the characters.
But especially if one fights enemies which do e.g. poison damage, one does not have to fear those enemies and their direct attacks, but only the e.g. poison pool that appears underneath them once they die, or which they summon during the fight. Never their normal attacks, always only, numb repetition style concept, the floor effects. The player character can endure the normal attacks no problem, but the floor effects are instant killer and one must evade them, even if one has 0.0 to 1.0 seconds to see them and react only. That is boring, and paired with the limited visibility due to other battle effect particles in the heat of battle, frustrating. I guess every player knows the own outburst along the lines “WTF killed me there?” - especially if the game mechanic did the damage of the floor effect before the floor effect visuals got rendered in the first place.
In the game’s options one cannot even turn the full particle rendering of own and/or enemy effects to e.g. a simple red circle (fire AoE), blue triangle (frost cone) etc. so that the visibility is enhanced. Another clear indicator of game design aiming at the fast and simple “player retention by frustration” instead of creating something fun and challenging, instead of creating a thought through skill/paragon and gear progression system, a fair mix of meaningful farming and strength growing, invested time and game controls playerskills.
6) Transmog & Mount Shop
6.1) Even shop customers who pay for the overly high priced shop products (transmogs & mounts) get only the least required quality for the products. While transmogs of armor and weapons are usually high quality, mounts (horses & cats) are often mostly a retextured instances of the same meshes - for full price nevertheless. Also shop products are partly not unique enough: you get similar items from ingame quests, stable vendor sales or other achievements.
6.2) The sales philosophy seems to be “pay and get lost”, shop customers are left alone with their purchases afterwards without any quality of life to manage them. There is no way of filtering and searching in the wardrobe. If buying several transmog sets (and/or farming them), it can get tedious to find the single items in the wardrobe. The transmog sets, as well as set-mounts and their set-saddles and set-mount-trophies are not grouped by set name ANYMORE (as it was at least for transmogs in the early seasons). Partly there is not even a similar name in single set items to find them and combine them anymore. Keyword is “anymore” as those set-lables were actively taken out of the game over the seasons.
6.3) There is no way of reading the lore/description of an item set or mount set anymore after the purchase, not even in the shop itself.
6.4) Weapons as well as mount armor/bardings and mount trophies cannot be tinted (e.g. to match the armor of the character). Also it would be a nice gimmick to turn off the head section of mount armors (for those it is plausible as they would not carry the neck armor for example) - at least for the cat mounts.