Tons of informations were released on an internal source during the night. Probably to let most of the GM know what's going on.
The expansion is Northrend, etc, etc.
We will fight Arthas himself.
Level cap is 80 (New skills, new talents, etc)
The expansion is designed for lvl 68 and higher players.
The Death Knight class will be available for both alliance and horde players and it seems to work on the same mechanics as Jedi in SWG, you can create them once certain criteria are met. (First hero class of the game)
The Death Knight will combine damage dealing and tanking.
Siege weapons and destructible buildings will be added to PvP
A new battleground will be available
New profession : Inscription. (allows the player to permanently enhance their spells and abilities and to create mysterious items of power to use, trade, and sell.)
More hero classes will be added in the future
Leveling from 70 to 80 should took nearly the same time as 60 to 70 (probably a bit more)
First zone will be Howling Fjord
We should have roughly the same amount of dungeons as BC
For more details, check wowszene.de
Of course, these informations aren't officialy from Blizzard, and you should trust them only if you're stupid/intelligent enough to trust this site. (Probability this is real : 70%, according to our fake-o-meter)