1) Disconnecting Off-tanks. If your internet isn't stable you probably shouldn't be doing an important job.
2) Disconnecting Healers. As (1), but I'm willing to forgive you if no other healers are online.
In fact, just about anyone Disconnecting. Except for the other Ret Pally, he can disconnect all he wants so long as he doesn't roll on gear.
3) Having to be a whelp tank to replace (1) as the only reasonably geared tank off-spec in the raid.
4) DPS in front of the Big Add, or anywhere near the Add's tank. Cleave = 5-yard chaining death.
5) Healers in front of the Big Add. See above.
6) Tanks not turning the big add away from (4) and (5).
7) Melee DPS without Enemy Cast Bars turned on, and hence standing in Blast Nova when it's a 5 fecking second cast. Seriously. Just. Argh!!!
8) Shoddy Phase 3 Transitions. Sometimes understandable, but Onyxia shouldn't be coming down in a nasty position if ranged DPS is told to stop at 41% until RL tells them to bring her down.
9) DPS Standing anywhere near the Whelp Pits during (8). If your whelp tank is pulling away from them now is not the time to rush towards them, if necessary stay in SW/SE until Onyxia is in the right place.
And the prime, number one thing I hate:
10) Wiping for 2 hours on a boss we really should one-shot by now.
Auxiliary thing I hate, not limited to Onyxia
Someone pointing out mid-raid where a specific member of the raid went wrong (in painstaking detail), going on about it for quarter of an hour without making any actually useful observations. A word to the wise everyone, you can only go on and on and on for so long until other raid members start to think you're just in love with your own voice.
This post was inspired by Wednesday's Onyxia raid.