I have personally (mostly) quit playing for a few reasons:
Huge discrepancy of player skill in team games. 1300 Elo players shouldn't be in the same game with 2300 Elo players. Makes the game wildly variable, even when numerically balanced.
Experienced Players room would fix my first point, but no one ever uses it.
Overall teamkilling problem. By this I mean: the game provides many ways to kill your team. We can ban those who TK, but then that brings up Licho's point: he thinks we are too aggressive with banning. I think he's wrong, but our overall ban/not ban policy is a bit murky and people lash out one way or another. I think if teamkilling mechanisms were removed, the problem would go away. Unfortunately, this is a very low-level design problem that would require massive changes to balance and game structure.
Skill growth stagnation. I have reached the point where I'm not getting any better at ZK. I like continuously improving whatever I'm doing, and I feel like I'm playing at my maximum potential skill in ZK. I wish I was better, but it seems like I have hit my ceiling.
UI problems. Seriously, the default UI is among the worst I've ever seen in a game. I spent a lot of time customizing my UI and every engine update seems to break my changes. As we all know, it's an amazing CPU hog and decreases everyone's frame rates.
AI problems. This is mostly driven by ZK's incredibly variable reload times. No other RTS has units with these massive reload times because it's wayyyyyy too easy to trick AI with it and render super expensive units absolutely useless. We've all seen the attempts at writing a no-overkill AA system but it's simply trying to patch the problem, not fixing the root cause. For example, an Annihilator can be tricked to shoot a 20m flea. A gnat can be used to trick the hacksaw. If they fired more rapidly, the problem would be reduced. I don't like games where it's more about tricking AI than out-playing your enemies.
Dirtbags. Enough said.