Luke's fight with Fake "Force cave" Vader is actually a fight with a Celestial Vader! In the original story, the swamp and the cave disappear, and Luke and Vader lightsaber it out in deep space. Vader even grabs a handful of stars to show how powerful the Dark Side can be. (That scene was apparently sponsored by LSD.)-- 100 Things You Didn't Know About The Empire Strikes Back [movies.ign.com]
There's some interesting production notes, but the first part is all about the differences between Leigh Brackett's original script and the edits Kasdan made. Brackett's draft is often credited for giving Empire the punch that nearly everything else Star Wars lacks. Interesting to me is that Empire has the lowest body count of any of the movies and yet is the clearly the darkest and grittiest of the bunch.