The Egotistical Priest has a good series of posts up attempting to somehow quantify whether Wrath has been an overall success or not. Of course, it's definitely a financial success, but has the game's second expansion delivered what both players and Blizzard expected it to? Vonya sets out to find out in what has turned into a three part post: you can find parts one and two on the site now, and part three is set to come out tomorrow.
So far, the answer is yes: while the area of Tradeskills is noted as less than a success (it seems to me, too, that tradeskills had more variety and options in Burning Crusade than their current state in Wrath, though that might be because we're only partway through the expansion cycle), everything else is noted as a win for Blizzard: they've really beefed up questing, balance has been intriguing since Wrath (and even if one class has rubbed you wrong, consider how many players came running back with the expansion patch to re-try their class), and of course, Achievements have (predictably) brought the game to new levels of addiction and given players of all kinds new things to do.
Vonya still plans to tackle instancing and raiding as the other two criteria for Wrath's success (and there are probably a few other ways you could test it -- lore? setting?), but by the reasoning so far, Wrath is a win no matter how you slice it. Blizzard has outdone themselves with the second expansion -- the only question is where they'll go from here.