So what are these abilities or spells for your class? I have a candidate for nomination, and that's the shaman weapon imbue Rockbiter Weapon. I can't imagine any situation where I'd want to generate huge amounts of threat as a shaman. Perhaps this is a leftover from when shamans could sort of tank back in the day? I honestly can't think of any other ability in the game that's quite so useless as that, but of course I haven't played every class, and there may be one out there.
What are your nominations? I'm not inviting class-specific QQ for so-called broken abilities; I really only want to know about things that just haven't a use in game. What can top Rockbiter Weapon in the most useless ability charts?