- Plainsrunning was a Tauren racial ability that was in the game before Blizzard implemented Kodos. After a quest, the cows got an aura (canceled with combat, underwater, or indoors, just like a normal mount) that let them move faster and faster up to a certain speed.
- Swirly ball was what the Rogue's Detect traps ability used to be-- a castable 3 minute buff that showed an annoying swirly ball that could be used to detect lag or just make noise.
- The old Hunter's Mark (as you all should know, this one wasn't long ago) was just an arrow, not the fancy schmancy (garish, if you ask me) deal we've got now.
- Baron Geddon's Living Bomb debuff used to be able to hit pets. Hunters would then dismiss their pets-- and resummon them in the Auction House to create carnage.
- The Hakkar virus was another debuff, this one from Hakkar, that did damage to anyone standing around the player. The debuff hit everyone in an AoE based on the target for a few hundred damage every few seconds for a few minutes, and passed on the plague. So players beat Hakkar, ported back to IF, and spread the disease around the world. This one actually made it to the media, and was used as a study for how disease spreads in a virtual environment.
- Captain Placeholder (my personal favorite) was a placeholder who went up while the ships between the continents were bugged. Don't miss the Lament of Captain Placeholder.
- Trolls used to have a "keel two dwarves in the mornin'" emote that got removed from the game, either for violence or drug references, depending on who you ask.
But the other interesting thing is that almost all of them are clear examples of emergent gameplay-- the devs didn't plan for this stuff to be popular, it just became so. If nothing else, they can learn from what happened with these, and (as with world events) bring them back in other forms. And that's a really interesting thought-- a game designed by the players themselves.