I asked this same question about Epcot:
What have you not done at Disney-MGM? I never saw the Narnia exhibit (it's gone, right?), I have not seen High School Musical Pep Rally, I never pulled the rope near the well by Indiana Jones Stunt Show and I never got to eat at Mama Melrose's Ristorante Italiano.
Last year I saw Sounds Dangerous for the first time.
What have you missed? Shows, shops, streetmosphere? Or is there a ride you haven't been on?