^ Well, you might be right about how Lucas perceived Brackett’s draft. I mean, at the very least, we know he was unsatisfied enough with it to rewrite from scratch. But if Lucas’ later Star Wars movies are anything to go by, there’s one aspect of Brackett’s draft that is quintessentially Lucas - even more so than Kasdan’s later drafts. I’m talking about the whole idea of how one falls to the Dark Side.
One issue I always had with Return of the Jedi was that I never really bought into the idea that Luke was at risk of turning evil. I love the whole Throne Room sequence with the Emperor, but I always questioned how the Emperor believed he had any chance of converting Luke. In fact, there’s little in the first two OT movies that really explores exactly how someone falls to the Dark Side. Yoda says that anger, aggression, etc. are the path to the Dark Side, that the Dark Side is perhaps quicker and easier, more seductive, but the actual process is left mostly to the imagination. Through later material, like ROTJ and the Prequels, we learn that the actual process of converting someone to the Dark Side basically involves really pissing them off and forcing them to act violently out of anger. This is what Vader does in ROTJ, and it almost works - but Luke manages to regain control of his rage.
The problem is that Vader being Luke’s father muddles this whole process. Luke loves his father - that’s the whole point - so the usual “Dark Side conversion therapy” doesn’t work. And not only does it not work on Luke, we have little reason to believe it really could have ever worked. But… if Vader wasn’t Luke’s father - if instead, Vader murdered Luke’s father - then Vader and the Emperor would have a lot more to work with in terms of converting Luke.
And this is exactly what we see play out in Brackett’s draft. The Luke vs. Vader fight in Brackett’s draft reads almost like a scene from the Prequels, with Vader taunting Luke about his murdered father, trying to make Luke angry enough to start using the Dark Side. And Vader’s tactics almost work - but Luke throws himself down the Cloud City ventilation shaft at the last minute. This whole sequence plays out exactly the way George Lucas seems to envision the “Dark Side conversion process”, as seen in later movies like ROTJ and the Prequels. So Brackett seemed to have a really good handle on the whole Force mythology and how the Dark Side works.
But when Vader became Luke’s father, this whole process became much more complex and muddled. If Vader’s goal was to convert Luke to the Dark Side, it makes little sense to reveal his identity as Luke’s father. From Vader’s perspective, it would be much easier to harness Luke’s anger if Luke believed Vader murdered his father, than it would be if Luke knew the truth that Vader IS Anakin Skywalker. This knowledge might trigger positive emotions in Luke, like affection or even love - which is exactly what happens. Now, I think ultimately the twist makes for a much more compelling story. (I’m not one of those fans that dislikes the big twist in ESB.) But it comes at the cost of muddling the whole Dark Side conversion mythology, and it makes the Emperor’s attempt to convert Luke in ROTJ seem like a long shot.
But the point is, later movies reveal that Brackett really nailed Lucas’ ideas about how the Dark Side works, and how one can fall to the Dark Side via embracing anger. And I think this was lost in later drafts of ESB. Vader has a few lines about “giving in to hatred” while fighting Luke on Bespin, but it’s all sort of deflated once Vader reveals he never actually wronged Luke (by killing his father), and in fact, actually IS his father. It also lessens the stakes during the final battle in ROTJ, because there’s much less the Emperor has to work with to make Luke turn evil. Still, I think it was worth it - because the twist in ESB is just too awesome to leave on the cutting floor.
There’s also some concept art by McQuarrie based on ideas exclusive to Brackett’s draft. So Brackett’s ideas at least got far along enough in the production process that McQuarrie created some art based on these ideas (or maybe it was the other way around and Brackett based ideas on some of McQuarrie’s available concept art). Ironically, even though Lucas famously hated Brackett’s draft, her draft gives me stronger vibes of “Prequel era Lucas” (i.e. the real Lucas) than Kasdan’s stuff.
Anyway, it would be nice to be able to read the 2nd draft of ESB, which Lucas wrote after reading Brackett’s draft. This would shed some light on the transitional stage of the script after Brackett but before Kasdan/Lucas ironed out Vader’s role. Unfortunately, I’ve never been able to find a legit scan of the 2nd draft anywhere online.