Some other additional musings:
The idea from the third draft that Princess Leia would look exactly like Luke, so as to communicate visually that they are two halves of one being and thus soul mates, may actually explain why Lucas didn't have Luke "get the girl" in the later films.
When Lucas cast Carrie Fisher as Leia, it was obviously not for physical resemblance to the blond Mark Hamill. As a brunette, Fisher better resembles the black-haired Leia (based upon Dale Arden from Flash Gordon) seen in Alex Tavoularis' storyboards. Consequently, she likely didn't fit the mold for Lucas's mental picture of Luke's girlfriend (i.e., a mirror image of him).
As a result of this dissonance, I would guess, in the second film Lucas decided to pair up Leia and Han, instead of having her get together with Luke as envisioned during SW 1977. Luke, meanwhile, would be associated with a girl who looked exactly like him, as per the idea from the original film.
However, since it would be extremely silly to just drop in another Princess as Luke's girlfriend, Lucas decided instead to make this new female character Luke's lost twin sister--taking the already-intended physical resemblance and running with it.
The plot thread of Luke's lost sister was meant to be fleshed out in the films following ESB--including the Sequel Trilogy as it was then imagined.
But after the exhaustion that overseeing the making of ESB led him to, Lucas decided to wrap things up with just one more film. The resulting compressed storyline meant that he had to find a way to introduce Luke's sister quickly.
He settled for making the lost sister be Leia--bringing his initial idea for Luke's "other half" full circle. Of course, this meant dropping the earlier intended notion of a marked physical resemblance entirely.
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On another note, poor Leia in the third draft actually ends up being the recipient of Vader's partial Force choke (used on Admiral Motti in the final film).
And later in the third draft script it's implied that Vader and two Imperial officers are the ones who physically beat her up, leaving her unconscious and bloody when the heroes break open her prison cell.
Paired with the Father Vader and Sister Leia retcons, that would have made for even more awkward viewing than the torture droid scene in the final film.