Things that bother me so far... always online DRM, they should have allowed an offline option while at the same time not allowing to transfer character from singleplayer to multi. Although they could allow it the other way around, from multi to single, that would be a good fix for the future, but it's totally not going to happen.
Action house looks like a pay to win service, not good... and who assures me that Blizzard is not going to place the best items for sale? huh?
The lack of character customization, not even hair color!
The lack of shiny loot, there are very few decent items which sucks considering that it's the main point of the game, no?