There's tons of stuff hidden in the game files as such. There's also tons of movies available online that show it off, such as the Emerald Dream (which has been in the game almost since Launch) and the original Outland / Hellfire Peninsula (most of which was reused in Burning Crusade).

For six years, there were multitudes of places that existed on Live servers that were covered up by the ever-present mountains that seperated zones. Twilight Highlands used to be a mish-mash of multi-colored terrain and crazy hills and valleys that went on for miles. One could stand atop it and look down on Grim Batol from above.

Hyjal is a shining example. It was there for the entire time leading up to Cata, at about 90% completion. It only needed quests and mobs added to it. If you study the original zone terrain for Hyjal using either map viewer, a Hunter or Shaman with "Far Sight / Eagle Eye" or via a private server, you'll see that Blizzard changed very little in the zone itself, with the majority of changes being done in Darkwhisper Gorge.

Point is, WoW has always been full of things that were planned, then scrapped. Or maybe they weren't really scrapped at all, maybe it's all been planned since the beginning, and that stuff was just partially done and not ready yet.