I'll start off with a few:
1) Lack of communication - Took them MONTHS to respond to pvp. At the end of December 2012, they release a 'pvp update'. This is AFTER they told us pvp not coming out in 2012 would be 'catastrophic'.
And yet they wonder why 20+ page threads have been created after a Jay Wilson TWEET about pvp. PVP IS endgame in an item-hunting game. People saying otherwise represent a minority and the majority has already - guess what - left.
2) Huge class imbalances - why can barbs roll extremely high main stat e.g. 90-100 vit on set items when other classes can only roll 30-100? Why do they get AR on 3 set items? Why do wizards tal rasha's set suck so much? WD having mana issues high MP? No problem - Ignore them! Why keep mentioning they are looking at nerfing monk's OWE and re-working them, only to provide damage changes and no actual skill changes to the 1.07 patch?
3) No end game content. Hellfire ring grinding is NOT satisfying content. Take a leaf out of PoE's book to look at how they've structured their end game.
4) I would mention the big D word. But I don't wand this thread to be deleted in 2 seconds.