Reserved slots, no I don't want to pay $5 a month to play on a server that might be full once per-day and be deserted for the rest of forever. I don't want to pay $5 a month for a server I only go on because there are no better ones, yet said server is overrun with mic-spammers and trash talking 14 year-olds with rubbish, schizophrenic admins who will let some people get away with murder while permabanning some guy for cursing once in chat. If that person isn't there than they don't deserve to keep out those who want to be.
Teams who have ludicrous amounts of a single class (usually snipers) and little to no medics and then flip out when they're losing.
Medics who DON'T DO THEIR JOBS! It is called medic for a reason, if you want to go all glory-hog and go battle medic at the expense of the people you're supposed to be healing than don't be a medic.
Annoying, sucky kids who try to be tactical leaders.
Rubbish medics who ignore everyone but their medic buddy, even if others are on fire next to them, they will not stop ubering that heavy.
FaN Scouts, that thing is so flipping annoying.
Class limits, I realize that they are a necessary evil, but when you have a crappy engie who builds all his buildings and never upgrades then I think someone who actually can play the class should have his spot.
The Huntsman, just the Huntsman, it annoys me how much damage that stupid thing does.
(just general complaint) Being on a killing spree only to be stopped by your mouse wigging out and then getting jumped by some engineer with a wrench.
Weirdos who treat the game like RL and flip out at losing and throughout the game whine about some issue that they have with the team.
(Been guilty of this) People who accuse people who play fair of hacking.
Stupid-ass teleporter placement (like entrance in the hay room in 2fort and exit a few feet away), no it is not useful for detecting spies, it's just stupid and annoying.
Having to be stuck in a crappy gimmick (low grav, high crits) server because your favorites are all filled.