People who keep playing classes they suck at and never learn from their mistakes, people who only play Pyro. Pocket medics kind of bug me, but usually it's only when they're on the enemy team. So I guess that would be a stupid complaint.
Another stupid complaint, people who only play sniper, and somehow get a headshot on EVERYONE (better than everyone else on their team). Like they've played TF2 for a trillion hours and only play as sniper. Sorry, I can't help but complain. But hey, at least I admit it's a stupid complaint!
Oh yeah, and MINI SENTRIES. Mini sentries are like a disease, and we need to find a cure.
People who complain about body shots. Seriously, I try and headshot when I can, but if I can help the team better by body shotting someone, I will.