It looks like you're new here. If you want to get involved, click one of these buttons!
There are a couple reasons I can give but they would be speculation like worgen being a playable race but I think the number one reason why this expansion will sell more copies than WOTLK and TBC and sell most copies of the mmorpg genre in 2010 is basically the who. Who could we fight that is more powerful than the lich king himself. Most people think the lich king dying is the end of WOW and no one can possibly be stronger than he is and still killable.
I think curiosity will be the biggest seller in WoWs next expansion. I think the main reason WoW is still going strong and WOTLK did so well because of the Lich king and almost every one that plays WoW wants a piece of him.