4) The class you play is underpowered / the class you don’t play is overpowered
It’s an RPG. Stats differ from level to level. Every character is equipped differently. How players choose to build their characters differs from person to person.
Given the sheer complexity of a game with five distinct classes, it's a surprise that the characters are as finely tuned as they are—and they're only going to be even more balanced in patches to come.
3) The lack of non-consensual PvP ruined competitive gameplay for Diablo III
Diablo 2 was never designed as a competitive game, and non-consensual PvP was a big reason why it failed as one. The implementation of non-consensual PvP allowed rogue players to barge into public sessions to deal harassment and death upon players who were simply there to play cooperatively against monsters.
While much was done to remove the element of surprise that non-consensual PvPers (or player killers, as they were known in Diablo argot), their presence simply invited players to quit the session and reform elsewhere.
2) Diablo III can’t compete against the likes of Torchlight II or Borderlands 2
Nor should it. Diablo III is a different game from Torchlight II and Borderlands 2, all of which have their own audiences and offer a variety of different experiences. There’s no reason for any of these games to want to be like the other, especially when they’re better off just doing their own thing and being excellent in their own ways.
1) Diablo III is a bad game
Diablo III may not have lived up to everyone’s expectations, but that hardly makes it a bad game. I’ve spent hundreds of hours in Diablo III, and I return to it every other day because the combat remains satisfying and the co-op gameplay is still enjoyable. Thanks to the addition of the new Monster Power setting, the game is even challenging once more.
It may not have lived up to the hype everyone had for it prior to its launch—it’s short, lacks extensive dungeons, and offers little in the way of story—but it’s still a good game for all it has to offer. It could be better, and given time and future expansions, it may well live up to the expectations everyone had for it.