1)People in trade who think being a pot-smoking retard makes them cool.
2)In leveling dungeons (vanilla/TBC), WotLK All-Stars doing their first/second alt who think that you can chain-pull+AoE just because you can do it in Northrend. Especially fun was three days of Mara chain-stunning mobs with idiots pulling the next three groups while I'm standing there with swirlies over my noggin.
3)Morons who won't wait for the tank to pull.
4)Stupid tanks that take too long to pull.
5)LFM: "X" Raid...PST with Achievement...I imagine the uber-L33T heros that do this one look like this IRL:
6)Some idiot spamming "Looking For W/E Profession," that don't add what recipe service they're seeking. This especially annoying when they're looking for some odd enchant that only Jushiro or Aestu is going to have.
7)Retards in trade asking to be sold something that is abundantly represented on the auction house...you know, like Crafted Light Shot.

People that beg to be sold something, then when you offer to sell it to them they're like, "Well, I paid Whatshisnuts less than that yesterday." Take your fat ass back to yesterday and buy some more from Whatshisnuts.
9)I don't have words for this, so....
10)The thing that has pissed me off most this expansion was my first healer in LFG Heroics. I got an Ulduar-geared tank, which was top-tier at the time. He goes, "YOUR GEAR SUCKS YOUR FAIL LOL." Then, despite my fail and suck gear...chain-pulled all the watchers. All three of them, all of which have the "no threat table" add. We wiped. He raged. Only redeeming thing about it was the mage that said, "You said his gear sucked then pulled everything in the room."
Your Pal,
Jubber