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.
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.