American Graffitti to me was more due to great writting and acting, and because that was a straightfoward script, taken out of life. There was no real direction needed, and you can tell by Ron Howard and Richard Dreyfuss and the other actors how at ease they were with those parts because it was like playing themselves.
With Star Wars, he was deriving on a mere concept he had, on vague ideas, of an adventure serial. While he wrote a good script, he was lucky to have a few things at his favor:
1- He got some great concept art and could do wonders with that. He didn't come up with Vader at ALL, it was his concept artist (whose name I forgot at the moment) who came up with that figure, and he merely threw in a persona to that image. He didn't have anything planned.
2- He was starting ILM at the best moment possible, with the best people possible, and I'm sure he wasn't fully aware of that.
3- He was lucky to have a consistent cast of unknowns, which did a great job. He was very lucky with Anthony Daniels characterization, would you enjoy SW as much if he had been an "used car salesman"?
4- He was VERY lucky to get Alec Guiness. If made 5 years before, or 5 years later, Alec Guiness would have refused the part, and Obi-Wan wouldn't have that much impact.
5- The problems that came with the production made the film better. Imagine if we had a stop-motion Jabba talking with Han Solo, as he wanted to do... It's like, when the shark for Jaws broke down, Spielberg had to shoot it more mysteriously, same thing here. He had to suffer to make it the best way he could, and it did.
6- He had GREAT editors who cut up the film in a great way. Those who have seen the cantina scene from the classic b&w workprint know what I'm talking about...
7- He was LUCKY to get JOHN WILLIAMS, who gave Star Wars a huge leap foward. Try wathcing it without the musical soundtrack...
8- He had Lawrence Kasdan and Irvin Kershner for ESB. Kershner KNEW how to make the second film of a trilogy, he KNEW how to conduct the movie, and how to make it work.
9- He had Frank Oz doing Yoda, which made the character belivable.
I mean, I could go on and on... Now, the prequels are WATCHABLE, they are actually good movies, they are quite well paced, well made, not as bad acted as some people say, they are enjoyable pieces of film, GL is a competent filmmaker, but that's it. He is a mediocre/good film maker, he can do some good things, he has great ideas, he is an excelent producer, he knows how to make money out of films better than anyone in the film industry, but that's it.
He is NOT a sci-fi genius like Asimov or HG Wells or Arthur C Clark whoever. He has no idea how to conduct such a thing, and it shows. I mean, a good example is the "parseks" controversy, on how a parsek is an unity of distance not time, well, GL didn't know! As mucha s he lie, saying that he threw that in because he wanted to show how Han Solo didn't know what he was saying, oh, well, how many film viewers know what a parsek is? He didn't know, and the sad part is that he lies to cover it.
He has absolutely no control of a consistence between characters and how to make a huge saga. He is NOT Tolkien. He dosen't try to keep consistence of how long a Death Star takes to be built so it synchs up with the timeline of the official cannon, because he dosen't care, he dosen't know how to keep track of those things...
I just wish Irvin Kershner had directed the prequels...