First, let's look at what D3 did well. The classes were all interesting and well-designed, even if they weren't very balanced. The art, sounds, and music were all absolutely fantastic. The game's presentation quality is overall top-notch. Where did it all fall apart?
The first big spot is the story. Most of the story was uninteresting. The nephilim plot arc is pretty dumb even if the protagonists themselves are quite well-written (OK, the female monk and demon hunter are pretty badly written). All the villains in D3 feel like cartoon characters where they constantly get foiled by those pesky nephilim and their stupid followers. The bad guys basically never win until the very end. There's never a big climax where you feel like everything is fucked. There's no real urgency. You know the climax is going to happen the way it does just like you know who Belial is when he's first introduced.
Honestly the biggest problem with D3's plot is that there are no twists. Nothing unexpected happens. Okay, Diablo is a girl. That was a bit unexpected. Also, Zoltan Kulle died (again). That was unexpected and unfortunate, if only because having him attack the player made no sense at all. I also really liked him (because everything he said was basically true). It's unfortunate that such a good character got axed in such a bitch way.
However, I think the game's plot failings (I haven't played RoS, so no commentary there) really pale in comparison to the mechanics failings. When the game was brand new and fresh, there were a number of really silly ways to make oneself invulnerable. Playing with friends was counter-intuitive (they made the game much harder and took away your followers) unless you played with the four-monk perma-invulnerable party. If you couldn't make yourself invulnerable and you couldn't perma-stun, D3 was quite hard. Most builds fell short midway through Hell (I had to escort a lot of friends past Hell Belial) and Inferno was an insidious grind unless you played one of the OP builds.
That's really the worst of it. You had to grind. Gear upgrades became slower and slower and the rewards became less and less. Just like in D2, you ended up grinding the strongest boss you could down over and over in the hopes of getting just one good drop. And those single good drops generally weren't for you; you had to sell them on the AH and try to buy something for yourself. Crafting sucked and crafting mats were garbage. Jewel combining and selling was annoying and not very profitable. The best money came from flipping good quality items that the "average" player needed. And as the player average got higher and higher, the value of items plummeted.
All this created a downward spiral that destroyed D3's loot satisfaction. People quit the game in Inferno, unsatisfied with the experience of the loot grind and unable to complete the full game. That experience soured the feelings of many people about D3, myself included.
Compare this to my experiences in other ARPGs though, I think that if I just stopped playing before hitting that endgame wall (if there was such a thing), I generally had a better experience. I'm having a good time playing Borderlands 2 and Torchlight 2, because I haven't made it to the "endgame" equivalent (I suspect Borderlands 2 will be fun even then, though). I think we expected the game to give us the same stream of positive reward feedback as it did back in Nightmare and it jilted us when we got to Inferno.
Of course the game is different now and I have no idea even how to classify it. I do know my best friend is playing a new wizard on Expert now and he's steamrolling the game, but I don't know how the loot curve works outside of that. I do know that there's alternate advancement now (a new way to add more positive feedback to the game) which I think is pretty good to help smooth the between-loot experience for max level characters.
I really do think that our expectations of a perfect game killed D3. Torchlight 2 was probably a better game at launch but definitely wasn't presented anywhere near as well. I think Torchlight 2 had its own mess of problems as well, but they're just less pronounced and less scrutinized than D3's were. If we look at any other ARPG's story, none of them are anywhere near the level of something like Dragon Age. Why are we so miffed at D3? Is it because Blizzard actually tried to add a story in? How do we compare it to the virtually nonexistent story in Path of Exile, or the original Diablo?
I think that's a really crappy reason to hate on D3, personally. It made mistakes, but if you like it, then that's fine. If you didn't like it or felt jilted, that's fine too, but there's no need to make it personal. I got my money's worth out of the game before RMAH transactions paid for it. I played for at least 200 hours. It was 60 dollars, but it was satisfying. Is it bad that the endgame was irritating when I played the game for 200 hours?