This all comes down to prior expectation. If this were Demon Slaughter: The Clickest, nobody would give a fuck unless they were comparing it directly to Diablo 2, which would be to some degree inevitable given how fundamentally Diablo 2 is a baseline for the genre. However, Diablo 3 comes under a hundred times the scrutiny in these areas because it's the direct titular successor, which means a lot more than just title. There's the expectation of continued story, themes, gameplay, depth, the online vs. singleplayer components, and replayability. Diablo 3 dropped the ball in a few areas (not necessarily wholesale, but in significant departures from the legacy that were perceived as negative by a large chunk of the fanbase). So yeah, the game itself isn't awful, and it isn't as though you can't sit down and play and enjoy it through the end. I did, more or less, despite how boring the last bit was. However, I played the original game and the sequel for hundreds of hours online with friends, and it was a very compelling and addictive experience. Online or off, this was the expectation for most. You don't blow one of the major elements that defines your game.
For me, I wouldn't have been playing the game that way in the first place. I don't have time. I got what I got out of it, and maybe I'll fiddle with it here or there a couple times in the future, see if I can beef up the character a bit more, but that will likely be about it. So this doesn't really affect me personally the way it would have, say, 7 or 8 years ago. Still, you can see the problems, and most of Blizzard's decisions here were poor ones. They've weakened the legacy of their own franchise, and people are annoyed. Just because you personally didn't want to get out of it what others did doesn't mean anything. There's a legacy to live up to, and while nobody says Blizzard employees need to go to jail or be executed for failing to do that, the consumer has every right to bitch. It wasn't the game it should have been given what came before it, and not just because every sequel should be more of the same, but because it's artistically obligated to innovate while still working toward the same ends. Clearly most people don't think it did.
EDIT - Which again really isn't my opinion. I don't really have an opinion on the life of this game because I can't in my situation. But I still see what people are getting at. And maybe it was unreasonable to expect any game to live up to Diablo 2. Maybe that game just hit at the right moment when the stars aligned, who knows. But I think some better decisions would at least have gotten them closer.
