So, you already know everything you need to know
about this World of Warcraft: Cataclysm trailer
. It introduces the newly ruined realm of Azeroth, two new playable races, new race / class combinations and new monsters. The level cap is higher, there are more skills to learn and more dungeons to explore. Basically, it's got all that wonderful stuff you want to see in an expansion.
All that you have to do now is watch.