His plots aren't all that bad.
Ah, I'd direct you to the writings of Harry Plinkett on that question. It's not just the plot holes, but really fundamental aspects of the prequel films: what is the Trade Federation, why are the blockading, what is the Republic exactly (The queen of Naboo is elected, but the senator of Naboo is appointed?), what is the fundamental cause of the rebellion, what exactly are the Jedi... These reflect on Lucas's really fundamental cynicism, and his inability to write characters as if they're intelligent agents that know what's going on, and his lack of faith in the audience to think about any of this stuff critically.
The first trilogy managed to keep all these balls in the air, but he didn't write those. George's writing work isn't really represented in any of the original Star Wars films. Larry Kasdan wrote V and VI, and though George's name is on the first one, he had a ton of help from Hal Barwood, Matthew Robbins, Will Huyck, Gloria Katz, Alec Guinness, de Palma, Spielberg and many, many others, who he failed to credit.
I dunno, he had a great original concept -- Flash Gordon meets World War II genre films -- and he saw it through to the conclusion, and he was the central person in those early films, but all the good stuff happened when he got out of the way and let the actors, Gary Kurtz, John Dykstra, John Williams and his wife Marcia do their magic. At some point in the 80s, after he banished Marcia and Gary and surrounded himself with sycophants, George must have thoroughly convinced himself that he did everything himself.
He doesn't give the actors feedback or direction - he expects them to bring the characters to life and flush out the nuances on his own.
Note that Michael Bay is known for this as well, and the results are very different. Not good, but different.
I strongly believe that Jake Lloyd was awful in Phantom Menace because of Lucas' directing style.
Jake Lloyd was terrible because George Lucas, himself, didn't know what Anakin was supposed to be or represent, what it was like to be him, what it meant to be a slave on Tatooine, or any of that. The character has no purpose in the movie but to establish that Anakin exists. Even if George were a "hands on" director, he wouldn't have had the slightest idea what to tell him. "Just sit in the cockpit while the battle happens."