I have previous expressed my burning hatred for whirlwinding mobs, and very few people are fans of getting feared into a nearby group of enemies.. But while leveling my pally, I've rediscovered my least favorite mob action of all time -- enemies that heal themselves when they're at about 20 percent health. These mobs also seem to conspire and wait until my Hammer of Justice in on cooldown, so all I can do is flail impotently with my big axe as they heal to 80 percent and laugh at me. I can also do a quick mana tap and arcane torrent, but by the time I think of that they're usually healed already.
As I write this, I'm fighting caster murlocs who are underwater, heal themselves, and run away when they're low on HP, which has got to be some kind of record for most annoying mob abilities combined in one. What is your most hated mob ability -- healing, fear, daze, or something even more sinister?