My biggest problems with D3 arent in the details, but in the core concepts. First, what i like about the game is the weight of the combat and the presentation. It seems im one of the few people who think the game is gorgeous.
It ends there however. Its overly simplistic and forgiving with its rpg elements, which is unforgivable since the combat (while has that a nice "omph" to it) is really simplistic. You cant, in 2012, have both a repetative combat system and stripped rpg elements, its either one or the other. Thats why a game usually is either an action/adventure game or an rpg, cause both are hard to implement at the same time, so a developer usually goes one route or the other. Here you have a game thats essentialy "left click until it dies", so it must have something else going for it to redeem that simplicity. It doesnt.
The narrative, as many have pointed out, is like something take from the power rangers. The dialogue is cringeworthy, the characters and plot uninteresting and predictable.
Besides the game being repetative, there is very little actual content. The Normal, Nightmare, Hell concept is outdated. You cant just take the same game, unlock a more difficult mode upon completion and call it value or replayability. Especially when the item hunt was so anticlimatic as it was in D3 comparing to D2. Imagine a person who just wants to play the game once; he gets neither challenge, depth nor story, since so much of the misguided d3 concept is based on diablo being fun "once it gets challenging". What blizzard is telling this guy is that he has to play through the game 1-2 more times to start getting to the good stuff. This might have worked in 2000, but now, with so many quality mmos and with gaming being so much further along then it was back then, a "lite mmo" with little content and broken systems like Diablo 3 wont cut it. Especially if you look how far rpg games have come during that time.
I cant believe that the developers thought nostalgia would somehow save this game from being mediocre. I remember diablo 2 and the item hunt. I remember farming cow level for items and dueling outside the town in act 1. But who doesnt at the same time realize and remember how low the standards were if you considered simply farming cow level and having nothing to do with your shiny items once you got them as something "fun".
This is exactly my problem with diablo 3. Its in many regards worse than d2, 12 years later. No pvp either, and ive not even touched upon the broken loot mechanics everything else thats wrong it in regards to difficulty. class balance and the AH. It fails as singleplayer game in almost every respect, and it also fails as a multiplayer game almost in every area.