To be fair, Lucas had great production design (from the late great John Barry), great practical effects work (now watered-down with some crappy CGI) and a supporting cast of great British actors to buoy his American neophyte actors and his awful dialogue. And occasionally, Lucas knows how to frame a shot or shoot a sequence.
Of course, Lucas did away with most of his advantages for the prequels, even as he tried to compensate for at least a few of his weaknesses (like the ringers who polished Revenge of the Sith's screenplay). Somewhere in those films, there are even some interesting implications and ideas to gloss the "original" trilogy with (like a religion whose "miracles" are based in a genetic mutation!). But that atrophying of his few talents/advantages is why the prequels' awfulness hit so hard.
I think Lucas' true genius was in consolidating so many pulp/b-movie cliches into one setting. Look at Episode IV. We start with a war/thriller angle (the pursuit of the Rebel cruiser and shipboard battle), detour into desert adventure (the C3PO/R2D2 travels) with a dash of Western (the Tusken Raiders as Indians, the Mos Eisley cantina scene), and get back to the spy/thriller before a rousing finale straight out of any war movie. The characters are just an amalgamation of different cliches, made interesting merely by the sheer oddity of combinations. Darth Vader is a robot/wizard/samurai, Han Solo a pirate/Wild West outlaw, and Luke a combination of hotrodding teen, young gun and brooding superhero.
And yet, most of the sci-fi and fantasy films have been ripping off these cliches for the past thirty plus years. No wonder s-f/fantasy is so creatively bankrupt.