For Star Wars, he had a terrible idea which slowly developed into something good. After some script re-writes, he got into something that would be interesting to see. In order to do that he had many influences that actually "worked" the script for him, Kurosawa as an example. He also had a great team and great minds working with him. In 1977 he had a visionary mind and a great criative vision.
Suddenly he becamse a multi-millionaire, and became distant to his creative mind and decided to focus on his business. He got old and lost all his experience. If you ask a 15 year old kid to tell you a story, and then ask that same person, 30 years later, to do it again, he'll not be as imaginative and creative as he used to be.
SFX, big budgets, bad luck, out of touch with today's world and a ego problem all contributed to what he is today. He might do some interesting non-SW films if he wants to, I'm sure he would do great drama films, or maybe a romantic comedy, but he has lost his touch for something as visionary and insanely creative as Star Wars is.