For one of the quests, The Abyssal Ride - Quest - World of Warcraft, in the Kelp'thar Forest, you are supposed to click on a little hook that lets out a line with an Abyssal Lure on it. For some reason, the Abyssal Lure is a hostile mob when cast by the opposing faction, and is a one shot kill. If you just repeatedly kill the opposing faction's fishes, they will never be able to get their seahorse.
This may not seem like an exploit, but this along with the fact that if you get attacked by the opposing faction in that area, they get hit for 45k damage by the water elemental guard that patrols. I am pretty sure that blizzard didn't intend for clients to not be able to complete a certain quest due to the opposite faction.
tl;dr - Kill ally Lures, never get touched because of patrol (infinite killing)
You also get to watch the alliance die when they attack you like fools, which is pretty funny.
YouTube - lolol