Rayless on the General Forums asks a question that I've always wondered about but never poked into; how exactly does phasing work? If you've leveled through the Death Knight starting area, done the Wrath Gate questline, or quested in Icecrown (and you should really do all three), you've had the opportunity to see Blizzard's most intricate phasing in action. However, Zarhym and Crygil are pretty cryptic on how it's done, and it's up to players to fill in the details.
In a nutshell, phasing is all about the information that's sent (or not sent) to your computer by the game server; Blizzard can toy with anything that's not client-side, affecting which buildings and NPCs you can "see" but not affecting the game's basic geography. I was surprised to discover that phasing has technically been in the game since launch -- ghosts and stealth are a form of phasing, as are (I would assume) the ghosts of Caer Darrow -- but the hugely elaborate set pieces of Wrath are simply a more complicated evolution of the same mechanic.
Given the success of phasing, players have been kicking around suggestions for instances or zones that could do with a touch of it, and Gnomeregan seems to be a pretty consistent pick. I'd have to agree, but I'd also add the Echo Isles (the Gnomes and Trolls have overcome their low-level foes by now, surely?) and perhaps Duskwood for starters. Is there any other zone or instance that you think would benefit from a little reality-bending?