WoW Insider has a gallery up of some of the hidden/obscure areas in Outland. Be warned, it does contain a few spoilers, so if you don't want to ruin the surprises of Outland, don't click it -
link.
I was thinking though, what's what's your favorite hidden/secret/closed area in game? Which ones would you like to see opened up in future?
I'd really like to be able to do some quests involving the
Dwarven Airfield above the Wetland and I'd love to see the
Old Ironforge and
Dalaran areas opened up to players - if only because they have pretty cool lore behind them (although the game could do with a bit more mid-level content now that BC is out). More details on closed zones at
WoWWiki.