List of stuff that grinds my gears:
People who type in Spanish in a ENGLISH ONLY chat room
6-year-olds who try to look tough by cussing at random people
when people participate in stupid, pointless arguments, for example: Metallica vs. Megadeth argument, Whether AWD drifting is "true" drifting or not, Battlefield vs. Call of Duty, etc.
when people drive dirty in gt5 in a CLEAN racing only lobby
when you get kicked out of a lobby in gt5 for stupid reasons (being better than somebody, because the host thinks you suck, people saying you were driving like a jerk when you weren't, etc.)
Show-offs (the mean ones)
and lot's, lot's more.