1. That random rage Pyro/Heavy
Especially if I miss a shot on them as Sniper, they're a pain in the ass to counter. They're two heavy-damage dealers, and if you miss just o n e shot on them, sorry buddy, you're off to hell.
2. Toxic/Salty people
This often annoys me more than you'd think. In my Discord server, I've already banned someone for excessive toxicity towards me and my friends, and about to ban another person for being overly salty.
3. Cheaters and Scammers
I remember seeing multiple cheaters when I play Casual (pretty common big duh). It ruins the aspect of the game, and even if they're kicked, they always rejoin. For me, a Medic main, extra points for me until they're kicked.