Every once in a while there will be a silly easter egg, or a cleverly hidden secret that everyone goes crazy for for a while, then it fades back into obscurity. What is everyone's favorite hidden content? I'm talking vanilla WoW - MoP, no restrictions. Quests, areas, events, pets, whatever!
My personal favorite has gotta be the Westfall Chicken. I'll never forget having to roll an alliance toon, level it, and buy stupid chicken feed just for Blizzard to make it available in Brill a few expansions later.
My second would probably have to be the Karazhan Crypts. There's just something awesomely spooky about them. I always secretly wished that Blizz re-used it for some Halloween event, or maybe a scenario or something.