I don't think Lucas ruined Star Wars, though I wish the prequels had been better (especially The Phantom Menace) and focused more on character than special effects. I have to admit that I did think he 'ruined' Vader for me. Even though I don't loath Hayden Christensen's performance, the Vader from the original films is diminished by Anakin's prequel portrayal.
Also, I didn't like all the Chosen One, virgin birth stuff, or Lucas making Vader the center of the Star Wars saga. The prophecy he came up with seemed to fit Luke better, and I have to wonder if it was all about selling the prequels that compelled Lucas to make it otherwise. Also I haven't cared for his tinkering with the originals. I didn't mind adding some special effects or a scene here or there to make them fit more with the prequels but sometimes I felt he was just changing stuff just to change it, to create more incentive for fans to dig back into their pockets.