Hacker group has announced that they will try to crack the online only aspect of the game, enabling non-official server play, which means you can pirate the game all you want.
Diablo 3 is simply described as Diablo series refined, and casualized. If you have played the previous games, it is pretty much the same in the core. If people wanted something new, then they have asked for a wrong game.
Casual'ness comes mostly in the fact that you can retrain your character all day long, and do not have to worry about skill point distribution. People view this as a bad thing in that it is less 'hardcore' about building your character, where in previous game you have to DELETE a character just because you misplaced a single stat point. However, I believe that that was stupid because it basically was artificially inflating the amount of play time, making people waste their time trying to find out the right build for themselves, and a character can be scrapped through a SINGLE stupid patch. Though, I will not argue that maybe the old selling point of the game was this pointless character building that some old fans enjoyed.
Auction house is something that I was quite worried about in the beginning, but as it turns out its pretty good. You do not have to have face to face trading anymore, and are able to find what you want (or may want) much easily. Besides the very unreasonable players (maybe vast majority?) quite a lot of items, that are actually beneficial, are priced appropriately so there is less worry about a person's bad luck for not getting the right drop (like myself for instance. LoL). Though, this may also roll into the whole 'hardcore'ness' of the game, as some people enjoyed this random gambling luck in the game.