If I let the Expansion alone speak on its merits, I'll say this:
The evolution of the game through this expansion is unprecedented. I'm not talking small scale features here and there, I'm saying almost every little thing has been revamped for the better. (I honestly didn't think it possible, but it is very much the case). the four new souls (3 of which have been revealed) are game changing in every aspect. you can be a melee healer, An AOE DPser using flamethrowers while moving around, and play Thor as a warrior) the new combinations alone are staggering and the Beta Forums are full of posts from people trying out new viable and fun specs.
The expansion contains one and a half times (to double) what I've grown to expect from expansions offered by other MMO's, and the stress test(s) on live beta servers have been Q&A fests with developers fielding a significant amount of questions and giving answers to a playerbase which has, for most part, been overwhelmingly positive in their feedback. (they rotated so as to take breaks but keep us engaged)
Hell, Its even impressed Puremallace (our resident loveable grouch) and that is really saying something.
Honestly speaking, Trion is proof that not all change has to be controversial, and while we are a very small community here, you can see from our posts and feedback just how happy we are with what they're doing with this expansion.
Objectively speaking? it is worth at least giving it a try. the worst that can happen to you from a choice perspective is that you'll find your heart is still in WoW and leave, but you will get your money's worth at the very least. for all my QQ, one of the nicest feelings from a gamer's POV is actually looking at your toons on the login screen telling yourself, "I can't decide which one to main because I can't tell which is the most fun."