I agree with you Jumpman--Darabont was hired as a writer, he wrote a screenplay and they didn't use it. Such is the life of a writer for mega-budget summer blockbusters. It is true that if Speilberg was excited to go and shoot that script right on the spot that Lucas probably should trust his director but ultimately it boils down to Darabont not being used to the process of writing. For him, he is simply used to getting his way for his projects because he is a pretty powerful guy that is usually allowed creative freedom to have his way. The same is assuredly true of Lucas as well, but its merely a clash of egos and Lucas is the one that hired him--if he says no, then he says no. Producers hire and reject writers on a pretty regular basis for this type of thing.

This really has nothing to do with prequels or whether or not Lucas is still talented. He is the executive producer, he is the one doing the hiring and firing and if an executive producer wants something changed--whether it is Lucas or anyone else--usually they win because they are one calling the shots.