But for me, berserker rage is way up there as a contender. That ability is easily one of the most archaic mechanics left in this game. I keep expecting it to get removed, but it just stays in, expansion after expansion. It's always been horrible design to allow one class to be so utterly immune to a major form of CC. Druids used to be immune to polymorph, but they changed that long ago to ensure the CC at least works against them. For some reason, warriors just get to be immune to fear, since creation of the game. This is especially stupid right now since fear has been completely gutted and is now the weakest CC in the game, but even when fear was strong, berserker rage was still just horrendous design.
Unbound Will, Ret's Hand of Sac, 10 thousand trinkets DKs have, Zerker Rage, etc. The only break I'd leave in game would be Nimble Brew, and ONLY for Windwalkers - Mistweavers and Brewmasters can suck on it.
Someone mentioned Faerie Fire, agreed.
I'm not so much annoyed at specific abilities ( although fucking trap is prolly #1 on that list ), as much as I'm annoyed at their design decisions. Sure, I get why they removed half of spells from certain specs - I was setting up my Frost DK yesterday ( I only had Unholy on him ), and there were no "clutter spells" I could mistake for as a part of rotation, it was as simple as adding 3 new spells to the bars to replace the Unholy variations on them, and that was it - but come the fuck on, some spells were iconic to classes and don't deserve to become talents and/or deleted. Howl of Terror, Mortal Coil, Psychic Scream, Cold Snap, Ice Barrier, Shadowfury, etc - all of those are iconic abilities that should have NEVER been fiddled with.
TL;DR - what i hate the most is MoP+ talent system.