George Lucas is great at telling a story and coming up with fun, charming, wondrous worlds, but good grief the guy doesn't know how to choose actors most of the time, second-guesses himself constantly, and is horrible at bringing out the best in an actor/actress.
Take Phantom Menace. Shmi Skywalker, Anakin's mom...wow, talk about lousy, FLAT acting! And little Anakin's acting pretty much sucked. That scene where Anakin has to leave his mother and walk away from her, oh man, if that had been Steven Spielberg or Peter Jackson, you'd be laced with tears, goosebumps, and you'd feel an emotional impact. With Lucas, you roll your eyes there.
Or take Natalie Portman. She's a wonderful actress, but in episodes I and II, she had lousy direction, input, inspiration, she fell flat as HECK! Finally in Sith she shined more. And look at Lucas's choice for older Anakin--Hayden Christensen. Oh brother! If that had been Leonardo DiCaprio, those 3 flicks would've rocked! That's a guy who can mix heroic with a dark, disturbing anger and confusion. Far superior.
And look at Lucas for dialogue: "I've got a bad feeling about this!" How many times did he milk that? Or the R2D2 squeeling sound....
The suspension of disbelief is thin in the prequels, and we see Lucas second-guess himself way too much.
Disney might actually improve the franchise IF and only IF they can get someone like Peter Jackson or Spielberg or of the calibre of Christopher Nolan. Otherwise, it could actually get way worse!