Star Wars started the entire idea of blockbusters, made Fox into a studio to be recokoned with, and is easily one of the most influencial movies of all time, so he'd almost get it even for that one movie alone. Not to mention THX mastering and remastering are the industry standard, ILM is still the measuring stick for any effects house, Pixar was started - and sold by, in maybe the worst move ever - by Lucas which, in turn, revolutionized 3D animation, and he completely revolutionized both music and sound effects usage in films as we know it (starting with Star Wars, of course).
He'd done so much on many different levels that both you and Kupek touched on as well as some none of us probably remembered. And that's not including the other media he's been rather heavily involved in, including LucasArts which was not allowed, in the beginning, that is, to even make licenced stuff (ie. Star Wars or Indiana Jones), which is how we ended up with Sam and Max, Day of the Tenticle, and Grim Fandango (among others). See that "lack of originality" post in Gamethought for an interview with one of the original LucasArts game designers (he just did Psychonauts) for a little more detail.
Of course, he's had a helluva lot of good people behind him to do all this. At the end of the day, he only knows how to write (that's debatable), direct (that's not; see American Graffitti), and edit films (he's considered by many one of the best editors around). Those are his skills, but he's smart enough to surround himself with people who invent these things and gives them the means, direction, and even provides some of the vision to do it (Lucas: "What if we cleaned up Star Wars to make the video much nicer to catch up to today's technological advances?"; voila, we have THX).
We may not agree with everything he does, but he is a brilliant businessman and a helluva leader. That's why he's a multi-billionaire with a ton of influence in Hollywood rather than a director working on the fringe of the studio system. Well, that and he had the foresight to include all merchandising and intellectual property rights to Star Wars as a franchise in his original director's fee, plus 40% of the gross :-) That would NEVER happen nowadays, but back then, that meant nothing.
