I’ll do my overall assessment of the game. Estimating my playtime around 200 hours post release. My highest character is around lvl 80 in the eternal non-season realm and lvl 60 in seasons. My mindset of D4 coming in, it will be my go-to game for the next few years.
Campaign: I enjoyed it, but have no interest in doing it again. The story with Lilith was very good. I enjoy stories where the enemy is well defined and worthy. The acts were uneven and some spots towards the end were too long. The beginning story was the highlight.
World 1: Never played. Always started on World 2.
World 2 and 3: Lots of things to focus on. Side quests and strongholds to gain renown points. Conquering dungeons around the map to unlock your codex of powers. Whispers of the tree adventuring is a good way to level up (post patch) and get crafting materials. World 3 adds Helltide, Nightmare dungeons, and unique items. Helltide is an event around the map that gives you another way to level your character and get crafting materials. Nightmare dungeons using Sigils for progressing your character and glyph leveling. I really like the nightmare dungeon concept. World 2 and 3 has a perfect amount of interesting things to do while leveling up and preparing for World 4.
World 4 and endgame: Here lies my main complaint about the game. World 4 is D4’s endgame. Root of my compliant, essentially there is nothing new in World 4. No new events, no new gameplay, no new zones, no new monsters, and hardly any new items to discover. A glaring gap in the game. There are only a handful of new ‘unique’ items to find which maybe 1 or 2 of those would be considered useful in your build. There is a new power tier of existing items called ancestral. There is one endgame boss and a seasonal boss. So in summary, things to do in world 4 is leveling and finding ancestral items while going through the same map, events, dungeons, as you did in World 2 and 3. World 4 must be fleshed out in every category.
Bosses: There are only three distinct bosses in D4. Lilith, the Butcher, and one world boss (that I like). I do love the whole Butcher concept. All other bosses are not memorable like we had in the previous Diablo games.
Character building: My absolutely favorite part of the game and why I am still playing today. Codex of power, Aspects, paragon system, and glyphs are the mechanics to tweak your build. I really enjoy figuring out my own build and there are many different ones for each of the five classes. I did follow a meta guide for one of my characters, but I have more fun doing my own. D4’s strength is having many playable builds and it has enough systems to make it engaging throughout level 1-80. 80-100 falls flat with nothing new to do or discover, so I have no interest for those levels.
Overall: My opinion the foundation is in place. Character building, the paragon and glyph system is fun to me. Disappointing, the endgame is not built yet thus my opinion it is an incomplete game at release that I expected. I do trust Blizzard will come through and flesh it out. I just hope it is quickly. Hearthstone, a game that I consistently play, is an example of them being committed long-term. They keep pumping out new stuff on a very frequent schedule.
To be specific, here is my list of what needs to be added quickly and D4 will become a complete game.
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Complete the endgame (World 4) with new quests, new zones, new dungeons, new events, new things to do, and add a ton of new items to find that further expands your character skills. Add new skills might be necessary. 
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Add three new classes 
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New distinct memorable boss fights 
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Each class needs about 20 new Aspects 
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Add new paragon boards to make character building even more fun