This is no ordinary expansion, this is a grand expansion.
WotLK has a new class: The Death Knight. It's a hero class which means it must be strong and it is and available to all races but you must have a character over level 55. It's very unique to any of the other classes and is available to all races.
The PvP of WotLK has undergone a massive improvement. The new PvP zone Wintergrasp is a much improved battleground than Eye of the Storm, 2 new arenas and Strand of The Ancients another battleground. PvP has been much improved in this expansion and PvP fans will be very happy.
The scenery is also quite spectacular and is amazing. It feels so real and you'll be awed. Also there have been lots of end game content including raids, dungeons and instances.
Overall with WotLK WoW is now the best MMO ever now. Now with great PvP , great PvE WoW is balanced and now everybody should be playing this. I've never seen such a good expansion ever.