Actually, Lucas is terrible at worldbuilding -- when he's in charge of that, we get Jar Jar and mitocloridians [sp?]. Trouble is he really doesn't have the SF mindset that extrapolates whatever into a universe. What he does have is a toymaker mindset that thinks the more weirdness you stuff into it, the better, and he doesn't know when to stop. His universe isn't built; it's cluttered.
He's pretty good at whizbang scripts and throwing out concepts when that's all he does with it (I say, having seen early SW4 iterations). He's not good at exploring the people beyond their surface reactions, or the world beyond throwing toys at it. Again, that's why we get crap like Jar Jar, who shows us how (Lucas thinks) we're supposed to react to all this clutter.
And he's not getting better; he's getting worse, because he's successful enough to be "uneditable" and can afford to totally indulge himself... something he couldn't yet do with the first three films. Plus back then he had real SF authors involved.
"I am your father" was character depth? Er, well, yeah, but SW fanfic explored that immediately after the first film, and IMO Lucas stole that idea straight from fanfic regardless of whatever he may claim. It sure isn't where the early concepts were going with the Vader character.
And about the first film, he originally insisted: "There will never be a Star Wars sequel. Sequels are made by people who can't think up new material." (That's from memory but it's real close to an exact quote.) Then when it became such a howling success, first there had always been three, and then there had always been nine...
Oh, the irony.