I really hate mechanics where if one person screws up, the entire raid is either instantly dead or might as well be dead because its a wipe. I'm looking at you Malkorok and Jin'rok.
I'm fine with the "one person can wipe the raid" thing if its a mechanic that the rest of the raid has to do, too. Malkorok, for example, is one of my favorite fights. One person can screw it up, but everyone has the same task, and its a very simple one at that. Puddles/smash/breath of Y'shaarj is a good way to see who's dumb and who's not. However, if it's a very specific task that's placed solely in the hands of one or two people (i.e. Magmaw chains, Siegecrafter belts, vehicle fights), then that's when I have a problem with it.
I hate to read all those buttons or just learning what the vehicles are for in general.
anything that relies on healers/dispellers to be clutch
The ones I hate most are the ones I want more of...
Single individual ability to wipe a raid...a la Flame Wreath on Shade of Aran in Kara
Love and hate that....
Personal most hated mechanic? Mounted combat a la Malygos. I can't figure up from down, left from right and
I was always, ALWAYS, the first to die. I outright stopped doing that raid when it was current b/c of those damn drakes.
Even if he was the stupid raid weekly quest I would not go. I still won't go even to farm for the mount LOL. Hate.
Anything that connects two people with a cutter beam mechanic and requires them to be smart for 10+ seconds.
My most hated raid mechanics are ones that randomly phase a single player to another area and then has them complete a mini-game where failure = raid wipe. Even when the task is simple, these mechanics are annoying because it can take so many wipes before everyone has had even 3 tries at it. I wish that when blizzard implements these things, they would make a practice it daily for people.
Boss healing up over and over and over.
Thankfully we won't be seeing that in WoD...I hope.
The entire Spoils of Pandaria encounter. I want to be with my entire team and damaging a boss that I can actually feel invested in killing. Spoils of Pandaria felt like annoying trash.
I personally am sick of the push mechanics ie Lei Shi, Jikun, Lei Shen.
Stuff that instawipes the raid because of 1 person making a slight mistake