I think I've figured out what bothers me about the PVE side of the game. The basic problem is that it's too random. You face an opponent much, much stronger than you are, and your only chance of victory is to play much, much better than the AI does. Which isn't necessarily that hard to do, as the AI isn't very bright. But winning or losing doesn't depend so much on how well you play as it does on how badly the AI screws up in that match, with an additional hefty dose of luck for which blocks spawn where and which random blocks get blown up. Having to wait until 10 minutes into a match to find out if an enemy hero's skill will instantly kill one of your heroes (so that you lose the match) or some irrelevant blocks (so that you win) gets annoying.