I saw a hint about this mentioned somewhere but I thought I'd start a thread on it here.
So I finished the Wrathgate quest line in Dragonblight today and I'm doing the followup quest "Fate, Up Against Your Will"
This quest sends you to Orgrimmar on a diplomatic mission to speak with Warchief Thrall.
At first I though I was instanced since there are Guards but they don't attack and there are no other NPCs or players.
Turns out it's the real Orgrimmar, just "filtered" using some new mechanic that Blizz has added to the game.
Other players can't see you and you can't see them unless they are on the same quest.
NPCs that you can see are altered by the quest.
The upside is you can fish in Orgrammar as long as you like without guards or players attacking you.
I will not turn in this quest until I have caught Old Crafty.
With a little luck I may catch the really big catfish while I'm at it.