For me, it's the people that say "I play for fun" as an excuse to run around and piss about, rather than to accomplish whatever objective or challenge the game has presented them.
I've noticed this most in Team Fortress 2 ever since the hats and trading came in. Whatever the jokes and general stigmas behind the direction TF2 has gone in recent years are, it's still a competitive multiplayer class-based team game.
One thing that I understand is going as whatever class you want. I'm all for that, I'd never want to force anyone to play as a class they don't want to just because we've already got four snipers and three spies, though I'd still prefer it if people could play a more complete range of classes...
What I really hate, however, are the people that sit around in spawn doing a whole lot of fuck all, meleeing their friend in a corner over and over for entire 20+ minute games.
I just don't get how that translates to "Playing for fun" when you spend an entire round in a corner holding M1, occasionally twitching the mouse. Spycrabbing had a lot to do with this, and people spend entire rounds single-mindedly walking out of spawn crouched and looking up with the disguise kit equipped, and I just think there is no possible way that could be fun to you in any way aside from the initial "Heee it looks kinda silly".
I'm a big player of Saxton Hale mode (and am a big fan of the Juggernaut-style gametype in general) and I notice it a whole lot on here. Engineers building as far away from the action as possible so they get long, eventless playtime, Scouts spending their time running from Hale as much as possible without touching left mouse once, those that hide until the end to 'friendly' with Hale, purely wasting time so those that died have to wait for them to act like idiots before getting another chance to play.
Basically, "Play for fun" to be is bullshit if you aren't deriving your fun from the gameplay. If you actively avoid the gameplay to have fun, join a trade/idle server where the whole point is to piss about. Just good luck getting out of spawn with snipers trained on the spawn doors.