Monetary wise, D3 is nothing short of a success. This is pretty much indisputable.
Actual game itself failed to the majority of players expecting a "Diablo" game. The list explaining why is rather long and it differs between the players (some hate the online-only, some hate the RMAH, endgame, etc.) and my personal list is rather long as well.
-Simplified Item Stats was a major step down from D2 (On this I blame the lack of innovation and making every form of dmg Crit-able)
-D3 is more of a goldfarm than an item hunt (Many factors from bad Item RNG to Global AH tool)
-Terrible post-login interface compared to D2 (and I promise this isn't nostalgia talking)
-Too tired to continue the list but those are the big ones for me
Now Blizz did provide something on D3 that I praise them for on this and just about all of their games: the smooth gameplay/visuals. ATM its the best I've seen compared to other ARPGs.