That Anakin Skywalker would actually suck as a character, rivaled in his cringeworthiness only by Jar Jar Binks.
That Boba Fett would be rendered completely uncool and be utterly deflated of all of his mystique and mystery.
That Darth Vader, king of all villains, would or even could be ruined by Lucas, let alone reduced to a whining, adolescent puss whose turn to the dark side is a grandious way of crossing his arms and stomping his feet at not getting his own way.
That Yoda would, much like Vader and Fett, suffer the horrible fate of de-mystification and be reduced a cheap, comic book gimic.
That the revered lightsaber handed down to Luke in ANH was actually the weapon of a Sith, used to slaughter kids, and not the heroic icon that we foolishly spent 28 years thinking that it was.