Blizzard have revealed the new expansion pack of world of warcraft at Blizzcon 2009.

What will be new in next expansion:

Level cap at 85
Horde gets the new race Goblins
Alliance gets the new race Worgens
You will experience that Azeroth has changed (Kalimdor and Eastern Kingdoms)
Lots of new dungeons and raids
100 of new quests
New secondary skill Archaeology
Experience old instances like Deadmines and Shadowfang Keep in Heroic difficulty
Guild levelling systems and achievements
The greatfather of blackwing - Deathwing is back

There will not be any new hero classes, mostly because it changes the game dynamics too much. But blizzard said that there will be new hero classes in the future.

Watch the new trailer here World of Warcraft: Cataclysm - Media