Anything that makes me gather a silly amount of pages for a book (Green Hills of Stranglethorn, Shredder Manual, etc). I'm a notorious packrat, which makes it nearly impossible to ever complete them due to inventory space issues.
Samophlange Manual in the Barrens also irritates me. The cave is densely populated, has frequent patrols, and the nearest graveyard is by the Crossroads. If you die in that cave, it takes so long to run back that EVERYTHING has respawned around your corpse. I just skip it these days.
Side note: I found more irritating quests Alliance-side than I have Horde-side, for some reason. I think they gave us Barrens chat to compensate
