D2, for me at least, was addicting but frustrating. The fun came from the slot machine reward from pulling the lever and tweaking my gear to min/max my lever pulls per hour so that I could tweak my gear more so that etc... This all came crashing down in 1.11 when runewords became the single institutional goal for my fun equation and I stopped playing. A lot of other things along the way like targetting inconsistencies, skill points, redoing your character every patch and how terribly boring it was to level from 1 to getting Whirlwind/Frozen Orb/etc... because spending points before that was a mistake, made the game very tedious in the parts between the killing stuff.
D3 looks and feels way better to play than D2, has a better leveling experience and has way more ways to make a bloody smear out of your enemies. I never got much satisfaction out of the act of killing most stuff in D2, but I do in D3 (aside: IMO, D1 did this better than D2 as well). It feels fantastic to butcher even the most common and simplest of enemies when their skull breaks off and goes flying by your PoV.
D3's tweaking of the min/max at first was about as bland as it can get. The first legendary revamp combined with MP settings changed this a fair bit and now I have to balance the circular equation of increasing damage out, decreasing damage in and optimizing hp gain with the fact that some slots are very good at two of those, but never at the same time. But it's still nowhere near as interesting of an equation as it was in D2. All defensive stats work in a similar manner and multiply each other. All offensive stats work in a similar manner and multiply each other. There's no trading an item with very high poison resist and physical damage reduction for something with high cold resist and increased block chance because that shores up your weaknesses better while maintaining similar offensive capabilities. D3 doesn't have those complexities in the end-game. It removed all the complexities of the early game (which I'm glad it did) but also removed them from the end-game where the only reason to stay there is to keep tweaking the interactions of those systems (drops and character enhancement). Add in the AH to the simple "hero score" (I'm using this as the term for the tweaking equation) and the process of improving that score loses most of its interest. Further combined with the lack of randomization in environments (events are awesome though, now that they made them worthwhile, and I'd like to see more reasons to venture into different places) and the end-game becomes even more linear.
I think I could liken it to the meta-game of BW or SC2: Everyone keeps trying to find the new, better way to do something. It happens over the course of years and yet what's optimal or best (not as in a single strategy, but in terms of scouting, reaction, how to read your opponent's choices, when and how to react to them or exploiting weaknesses, etc...) continues to change bit by bit. That's what D2 had, and it's what D3 doesn't. The equation has been solved, and the solution is widely known because there were too few variables.
