Things like that are really depressing to read, but I don't think it's fair the way many people now say that Lucas just got lucky the first time around, or the other trend in SW fandom which is to try downplay Lucas's involvement in the Other in favor of the other writers and directors.
I firmly believe that the George Lucas of the 70s and early 80's was a genius and a visionary, someone who changed movies forever both with his movies (and let's not forget he also gave us Indiana Jones) and his contributions to movie technology with things like THX and ILM.
I think it's more a case of a young visionary losing (like so many artists) that spark and passion, probably due to his much publicised personal problems during the making of the OT sequels and ther fact he almost had a breakdown making them.
Funnily enough after posting that I went on a bit of a Google adventure, reading about Lucas's directing career. I read a few articles which backed up what you've said - that he was experimental and insightful when he was younger. I've never seen THX or American Graffiti so I couldn't say, but I prefer that version of events. A tragic loss of creativity seems somehow easier to swallow than a lack of any to begin with.