The original Star Wars trilogy had a very distinctive beginning, middle, and end. How do you write about Luke Skywalker and keep it interesting when he's already saved Vader and crippled the Galactic Empire? Easy. You make up a sh*tload of characters and undo everything that made the series great. Here's our tribute to the Expanded Universe's biggest crimes against the franchise.
In The Movies: Vader's character arc crystallizes in a single moment. Inspired by his son's goodness and faith in him, Vader picks up the Emperor, the personification of corruption in both himself and the galaxy, and throws him into a conveniently-located bottomless shaft. That thing was just a lawsuit waiting to happen.
In The Expanded Universe: But wait
what was that about clones? Ah, that's right. It turns out the Emperor has a whole ship full of younger hims, just waiting to spring into action if the old him ever died. (Why did he bother hanging out in that crippled old body in the first place? We don't know, but Space Viagra must be super effective.) Vader's final sacrifice, his ultimate act of defiance, now comes off as a minor annoyance. He might as well have stepped on the Emperor's robe while he was walking down the stairs.