So I read the full ESB draft script by Leigh Brackett. Here’s some random musings that occurred to me when reading this:

This draft is not very good, especially compared to the final product. The overall plot is basically the same (Rebels are hiding in a secret ice planet base, the Empire finds them, Luke goes off to train on a swamp planet, Han and Leia survive a space chase and end up at Lando’s floating city, get betrayed by Lando, and Luke shows up and confronts Vader). Despite having the same plot broadly speaking, there really is not a single scene in this draft that would be recognizable if actually filmed.
The whole thing feels way more campy or whimsical than ESB. For example, in ESB, Echo Base looks like a practical, functional military base. But in the draft, the base is inside some kind of whimsical ice castle, with secret invisible doors in the ice.
The script spends more time on a sub-plot about ice monsters (the Wampas) attacking the base than on the actual Empire attacking the base. Ice monsters attack the base, killing many people, and forcing the rebels to evacuate. While the rebels are evacuating due to the ice monster attack, the Empire also coincidentally shows up at the same time. The ice monsters are almost supernaturally powerful. It’s really stupid.
Luke still acts like he does in Episode IV. He is portrayed as immature and inexperienced, compared to the suave, confident and experienced Han. The script implies that part of Luke’s motivation for becoming a Jedi is basically so Leia will take him seriously as a potential romantic partner.
There’s a lot more exposition about the state of the Galaxy. There’s a scene where a Rebel commander explains that after the Death Star was destroyed, many systems joined the Rebellion, but the Imperial navy is still very powerful, and is currently spread thin around the Galaxy pursuing rebel cells. This kind of exposition is missing in the final version, requiring us to infer these background details ourselves. (Also the final version seems to imply Echo Base is the only rebel base.)
The Force is way more vague. At one point, Darth Vader can force choke Luke from thousands of miles away merely by sensing his presence. Luke only barely escapes by jumping into hyperspace. (In the final draft, Vader force chokes Ozzel from a distance, which is also a bit iffy - but at least it’s implied that Vader has to at least visually see his target in that case.)
In this draft, it’s possible to summon a Force Ghost by calling out “By the Force, I call you!”. This is how Luke summons Ben Kenobi’s ghost. This is utterly horrible.
There is way more emphasis on the threat of Luke turning to the Dark Side. Since Vader is not Luke’s father in this version, Vader taunts Luke about how he murdered Luke’s father when they fight in Cloud City. This makes Luke very angry, and he begins using the Dark Side. It’s implied that Luke is on the verge of becoming evil from repeated Dark Side usage, but then at the last second Luke stops fighting Vader and throws himself down a shaft (similar to what happens in the final version). I like the final version much better, but it occurred to me that one consequence of merging Vader with Luke’s father was that the threat of Luke turning evil never seemed plausible. In the final version of ESB, Vader is Luke’s father and has nothing to taunt Luke with to inflame his anger and turn him towards the Dark Side. But in this earlier draft, since Vader murdered Luke’s father, Vader is easily able to make Luke angry, causing Luke to embrace the Dark Side.

Interestingly, there’s also some elements in this script that seem to have been recycled as concepts that appeared in the Prequels, decades later. Some examples:

Darth Vader (who is not Luke’s father in this version) more explicitly tempts Luke with the power of the Dark Side. In fact, Vader specifically tells Luke that the Dark Side will enable him to “keep those he loves safe from harm”. Earlier it is established that Luke worries a lot about Princess Leia’s safety, and Vader exploits this when tempting Luke. This dynamic is pretty much identical to the way Palpatine tempts Anakin in the Prequels.
Cloud City has elements that were almost certainly recycled into Kamino in Attack of the Clones. In the draft, Cloud City (called Orbital City) has tall, pale-white-skinned, native aliens that ride “giant manta-rays”. They also use “pneumatic darts” as weapons. This was obviously recycled as the Kamino aliens. (But the recycling feels a bit clumsy - the Kamino aliens don’t really fit the water-themed environment they inhabit, whereas these native cloud aliens were supposed to feel like airy warriors that dwell in the clouds - hence they use primitive darts. But why would the advanced Kamino aliens use darts?).
Yoda (in this draft named “Minch”) uses a lightsaber, and at one point has a mock lightsaber duel with Ben Kenobi’s force ghost. This is the corniest thing ever. I don’t understand how Brackett wrote this thinking it would be filmable. (I don’t know if this scene specifically inspired Lucas to have Yoda fight with a lightsaber in the Prequels; it’s probably more likely that Lucas just thought it would be cool.)