Basically...he's had two THX and AMERICAN GRAFFITI. Both STAR WARS and INDIANA JONES were serial storylines rehashed. Don't get me wrong...he did an incredible job...but it ends there.
I think Lucas' downfall came in owning such huge, innovative companies such as ILM, THX, Skywalker Sound, Pixar, etc. When his personal life went to hell after Marcia left, he griped the reins of his companies tighter. You can see his obsession with control and perfection in his directing. Directing for Lucas is now more of an exercise in creating 'perfection' than an artistic representation from within himself.
I, for one, can't wait to see something new and different from the guy. I think there is a good filmmaker still there...although I didn't really see evidence in SITH of it...most certainly not in the last thirty minutes. It was typical Lucas cut-ups.