The new instances are awesome! Something for everyone from casual gamer to hardcore raider.
On the other hand, the dramatic increase in achievement points will give you something to do for the next year. And the increase in instanced zones will also keep you busy for a quite a while. The numerous new instances aren't merely hack and slash dungeons. Some require a bit of thought while others require quite a bit of group coordination, similar in some ways to a mini-raid.
A note on difficulty - I rated the game as "hard" but there is plenty to do for the more casual gamer, but other aspects of the expansion will require more concentrated effort.
Update 6/4/2009 - After over six months of playing this expansion, I still love it. The classes seem more balanced now, and the most recent updates have added good new content. The next Live Update should also prove interesting. I hope you give it a try or come back if you've been away from game for a while.