Yoou need to realize that WoW is nearing the end of it's lifecycle. Now before I get flammed to high hell, let me explain.

WoW was released November 23, 2004 so when WotLK hits, itll be approx. 4 years old. By the time the next expansion hits, it'll be a 5 year old game. MMOs typically drop off drastically after that point. Sure, some people will stick around until the bitter end. But the majority will move on to bigger and better things.

Dont believe me? Look at Warhammer. That game is mediocre at best, but it still managed to make a rift in the MMO space because people were so starved for something different. There a number of MMOs that are set to be released next year and you can bet your bottom dollar at least one of them will have the polish WoW has (Check out Curt Schilling's 38 Studios). Blizzard knows this, and that is why they started developing their next MMO already, and Im sure by next year we'll at least have a name to go along with it.

The point is, they doint have time to sit on WoW anymore. They need to get expansions out on a regular basis to ensure people stick around to see the content. Otherwise, by the time you DING 80 in Northrend and are sitting around for a year and a half doing nothing, you'll be tempted to quit.