An eldritch abomination, a horror that cannot be borne. I'm talking about the Horde side Onyxia attunement quest chain. To this day I can't think of a single quest chain or attunement as bad, not even the ridiculously labyrinthine series of attunements required in The Burning Crusade, because the Horde attunement had the added difficulty of a turn-in NPC who moved around. Rexxar's daily wander up and down the road from Desolance to the top of Feralas and back made finding him for this quest absolutely nightmarish in the time before flight. All you could do was ride up and down hoping to find him, and doubly hoping a group of Alliance hadn't killed him on their way to or from something like a Dragon of Nightmare (one spawned in northern Feralas at the time). It was seriously quite insanely irritating.
So yes, it must never be brought back in any shape or form. To do so would be abominable.
What's your biggest "Let this never happen again" moment in WoW?