So, they are in a good place now, then? Item budgeting should be really hard. That’s a sign of a good system with lots of worthwhile options.
All of these are present in D4 also.
Creating a compelling game with that “one more turn” feel means creating lots of overlapping arcs, so that when you finish one there’s always another about to finish. D4 does a good job of this early on: you are leveling up, finding higher-power items, leveling your codex powers, finding needed uniques, completing capstones, and leveling glyphs / unlocking higher tier NMDs. You also have the season journey and the seasonal reputation track to give short-term successes. Originally, they had renown and codex dungeons for this purpose, but everyone hated it.
The issue D4 runs into is that you hit the end of leveling really fast and then quickly get to the point where your only loops are finishing the specific content you are running and masterworking your items. Even items drops tail off except for ubers.
In theory, runes gave two separate things: extremely rare items to chase, and a way to steadily progress towards a desired item. In practice, I think they actually played a lot more like chasing Uber uniques does in D4.
The xpac is separating paragon as its own levels. I think this gives them the opportunity to extend the process of leveling paragon to max while incorporating some of the other loops at the same time. Then you get 4 very clear loops: (1) the current piece of content you are completing; (2) completing the journey and reputation track steps; (3) leveling up; and (4) getting better items. There’s also a meta-progression to (1) where you are able to unlock (and are good enough to defeat) higher- and higher-tier content.
The main thing missing once they do this would be an overhaul of NMDs and glyph XP. I think the goal of that should be to fit them into a smooth content-progression system where instead of leveling the overworld and helltides and other content based on your level, they level it based on your highest-completed NMD tier. And then they can make it so that you get glyph XP from chests and whispers based on the level of the content you are facing. Who knows, maybe we will get something like this in the xpac.
Of course, I’d still like to see them implement a more exploratory dungeon run, where you have reasons to hunt around the dungeons for all the cool stuff and you can unlock deeper and deeper levels (and come back to them later, allowing you to log out in between). But they won’t need that for the sake of creating compelling game loops.
D4 eventually gets to this with NMDs and Pit tiers (and presumably with the Infernal Hordes), but the overworld scaling muddies it for everything except the capstones, and pushing NMD tiers isn’t really that central of a thing. Another case that will be improved by shifting to have you get into the endgame at level 60, and more slowly level your paragon while pushing NMDs high enough to unlock Pits, then pushing Pits.
I don’t think this is a requirement or a particular reason for D2’s success. In fact, having ways to target farm specific desirable items is one of the things that makes D2’s endgame interesting. Exclusive rewards are not that different from strong target farming zones. I do think it’s a good check on the content types in general, though: if an activity is only being done because it has exclusive rewards and would therefore never be done if you gave small amounts of those rewards for the other activities, then the activity needs some dramatic improvement.
It doesn’t make up all your power in D4 either - that’s D3 you are thinking of. Gear is the goal in D2 just as it is in D4. Top level gear makes you way stronger than not having it, but you still have inherent power in your skill tree and paragon. That power is then multiplied by the gear, which means that having no gear makes you much weaker. If you don’t think that’s true, try doing a corpse run in D2 without grabbing any backup gear.
You left out the key thing that D2 has that D4 does not have: problems your character has to solve at the higher levels that are different from (and in addition to) simply raising the thresholds of the lower-level problems. D4 tried to do a little of this, but the problems they introduced were hard CC, healing monsters, fields that shut off ranged damage, and off-screen spike damage. All of those are annoying or impossible to fight and the item / skill system simply wasn’t designed to fight them in a reasonable way. They need to take a hard look at how they want difficulty to evolve and create a new layer of challenges that builds actually have multiple different tools against. Not just “stack life and DR because you take more damage,” but rather “stack armor or max resist or DR or life.” And that same tradeoff for other problems like CC or monster healing or monster resistances, etc.