For me, it's young children. You find them in CS:S, you find them in Halo 3, but I don't play those, so I mostly find them in TF2. But what really ticks me off is that I find children in Modern Warfare 2 of all games, and to find kids who from the sounds of their voices can only be about 12 years old in a game that in the UK has an 18 Certificate on it is simply not right.
And why do these children annoy me? Well, they have no idea of the appropriate volume to speak at, they try and order you around, they call everyone else n00bs and hackers, they tend to resort to using swear words and racial slurs at a speed faster than that at which Yahtzee speaks, and whenever they're doing badly, they try and blame every single person but themselves, often calling people on the other team Hackers when all they did was headshot them. It's really annoying.
/rant
And why do these children annoy me? Well, they have no idea of the appropriate volume to speak at, they try and order you around, they call everyone else n00bs and hackers, they tend to resort to using swear words and racial slurs at a speed faster than that at which Yahtzee speaks, and whenever they're doing badly, they try and blame every single person but themselves, often calling people on the other team Hackers when all they did was headshot them. It's really annoying.
/rant