Everything but the Thrawn trilogy and the Jedi Knight and KoToR games.

By allowing such terrible writers to impose their terrible ideas onto the great potential of the Star Wars universe in such a convoluted and contrived manner, Lucas completely alienated millions of potential fans and only helped to solidify it's stereotype as factor 11 geekdom. Star Wars went from pop culture gold to sweaty nerd fiction in less than a decade.

If we were going for specifics though I'd aim most of my dislike towards how the history of the Sith and Jedi were handled. The Sith should never of been a race with a such a convoluted back story, they should of simply been the anti-Jedi, a group of corrupt force wielders who used their powers for nothing but control.

I don't even like that Palpatine and Vader were suddenly turned into Sith. The fact Lucas borrowed so heavily from the EU shows nothing but his complete lack of creativity, but now I'm starting to go into my hatred of the prequels, and that would take forever.