Then only thing that Diablo 3 dev team did wrong was to completely destroy the dark gothic setting of the original Diablo. All else they did as the fans asked. Fans wanted some major problems from Diablo 2 addressed and Willsons' team did exactly that. It turned out that it was not for good. Fans wanted harder game, but thy didn't like it so Blizzard nerfed it. Fans wanted to do something different than Baal runs, Diablo 3 gave it to them (rare and champions packs) and the fans didn't like it. Fans wanted more choices of endgame items, Blizzard gave them rares to improve randomness and diversity but fans did not like it and wanted legendaries back, again making them the most sought items (as runewords and uniques were in D2). The list is long.
So the whole development of Diablo 3 was centered around addressing the issues of Diablo 2, but in the end the fans didn't like it and wanted the old issues back. If anything ruined D3's launch, it was too much listening to the fans.
The main issue is that fans don't know what they want. In most cases a good idea turns out to be bad when actually implemented.
Three things that ruined Diablo 3 : the auction house, always online DRM and fixed skill tree( no point allocation in different branches ). I cannot think any of them being supported by fans.