Been reading this book (It's a bit old, came out in 2013, right after the Disney sale so it gets a bit of a mention) but here's some interesting tidbits from what I've read so far.
-Although he'd later change this for the prequels pretty much, Lucas in the story meetings says that the force is can be for everyone, but it's only the Jedi who really focus on it and use it. He also states that Yoda is in fact, not a Jedi, but is more of a sort of religious elder, and Palpatine has a similar role for the dark side.
-Alec Guinness was originally promised a larger role-he wasn't too happy with only standing still in ESB. While the original script by Lucas gave him more to do as Obi-Wan would come back to life and even chat with the Emperor and Vader a bit, this was rolled back to him being a ghost again, although he was given a bit more to do than in ROTJ, including being able to move around and sit down. (Force ghosts being able to interact to the physical world is something that the sequels picked up on too I guess).
-Lucas describes the whole Anakin backstory, which lines up with the prequels to a degree except with the mother, who lives for a while on Alderaan with Leia.
-Ralph Mcquarrie wasn't too pleased with Jabba ending up being a slug/worm creature.
-Speaking of Jabba, Lucas's primary inspiration seems to be the Fat man played by Sidney Greenstreet from The Maltese Falcon.
-There was some talk of killing Lando off. I wonder if this in part led to the internet myth that the Falcon/Lando would die at the end of ROTJ.
-Lucas just decides to ditch the concept of Imperial city planet Had Abbadon, citing costs and other concerns. Of course the planet would be reimagined as Coruscant by the EU and Lucas himself in the prequels. Kasdan didn't agree with this, citing in part that the finale would now seem a bit too much like the original film.
-Kasdan wasn't too happy with the Ewoks either.
-Originally, Leia was not going to accompany the rest of the group to Tatooine but do her own "Forest moon" mission, and the movie would open with her doing the shuttle mission alone. Other than that, the Jabba part of the story is pretty much the most intact part of the original concepts for the film.
-At one point they thought of the Rebels turning the Death Star on Had Abbadon!
-They thought about using Vader's star destroyer for the opening scene, but decided to just go with the regular one instead for some reason. The old Marvel comics adaptation has this. However in the finished film Vader's destroyer doesn't show up until the Tyderium mission.
-They also mused having Luke become the new Vader for a bit, but Lucas wanted a happy ending.
-They also mused having Luke be the new peaceful leader of the Galaxy, but Lucas also made it clear that Luke would be content with simply being a Jedi.
-Jerjorodd. The Death Star commander-who is given a minor role in the final film is a major player in earlier drafts as a rival of Vader's, and at one point was even considered to be an alien, with a look that seems a bit like a mix between the Emperor's advisors and the Trade Federation guys from the prequels. The concept of Vader having a dangerous rival would of course wind up in some EU, such as Shadows of the Empire, and the first Marvel Vader comic. Also the relationship between Kylo and Hux in the sequels. Also, funny thing is in the earlier scripts he is the one lecturing Vader about the Death Star! He even gets the line "The Emperor does not share your optimistic appraisal of the situation". In the film's deleted scenes, Jerjorodd does get some more stuff to do, and a tense scene with Vader, but nothing like the original draft.