1. Blatant team stacking. TF2 is at its best when matches are a challenge for both sides, requiring players to utilize good teamwork to prevail. I can't stand it when all the skilled players deliberately join one team and faceroll the competition. It's frustrating if I'm on the losing side and boring if I'm on the winning side. It indicates to me that they're too cowardly and insecure to handle the prospect of losing, so they ruin the experience for everyone else. Go play a match against bots or a single player FPS if you're that desperate for some empowerment. Bonus points if they re-stack after teams have been scrambled and/or respond with "lrn 2 play n00b" when you call them out on it.
2. People idling/trading in a "normal" server. Look, I get it. People like collecting things, and Hat Fortress 2 has plenty of cosmetics to collect (some of them don't even clash with the game's original art style). But if all you're going to do is idle for item drops or trade for hats, then
join a goddamn trading or idling server. You know, the plethora of servers that cater to such things so you aren't dead weight to players that actually want to play the game?
3. MvM. Don't get me wrong, I love the concept, but more often than not it's just an exercise in frustration. Chances are good that you're going to join a match that's plagued with teammates that can't even beat Doe's Drill.
I'm probably going to sound like an elitist prick here, but MvM is catered to TF2 veterans like myself. If you want to be successful, you have to know what you're doing. And you HAVE to work together to prevail. I can't tell you how many times matches are lost because everyone just runs off doing their own thing. Put simply, if you're new to the game or aren't willing to be a team player, then you have no business playing MvM. Until Valve comes up with some kind of "easy mode" that allows more freedom of play (something I'd welcome), that's the unfortunate truth.
And of course there's always that one player who never hits F4 to skip the countdown timer. Every. Single. Time.
4. Engineers building and upgrading in spawn before moving up (assuming they do move up). The battle's over there, numbnuts! The team might as well be short a player for all the help you're being. I'd rather see a level 1 sentry on the front lines than a level 3 one at spawn. Remember, all it takes is one backstab or headshot while carrying a building to sabotage all that work.
This should be required viewing the first time someone plays Engineer:
5. Smartasses. You know who I'm talking about. People who go out of their way to try and stir up trouble and cause fights. There's always the Mute option (which is incredibly useful), but sometimes that isn't enough. There have been a few times where I've had to switch off
all voice communication because the entire server was filled with douchebags.
On a related note, I've had sprays disabled for years now. One can only take so many goatse images before throwing in the towel.