Though I agree with bwayphreak that the Broadway production of Wicked has held up particularly well compared to other long-running shows, I think that their ensemble has become more and more robotic each time I return to the Gershwin. Seems plenty of them are long-standing, loyal alums who have been there forever, which is great, but I think it takes a little bit of a toll on the show's energy.
Also, this is rather specific, but I think a good chunk of Wicked's "spark" was removed when they took out the large, looming, circling wicked witch's hat from the top of the show. Since then, I just haven't felt the utter marvel it had on me 12+ years ago (please note I say all these things as a diehard Wicked fan!).