This issue got considerable coverage in the fanboy community immediately after Ep. 1; specifically, Qui-gonn (the fourth Jedi we see die in real time, and the third light-side Jedi) is the first good guy who doesn't evaporate.

Then, they proceed to kill a bunch of folk in Ep. 2, and they don't, either.

The bit at the end was supposed to be Lucas explaining why Yoda and Obi-Wan get to evaporate, and the rest don't.

Not that that explains why Vader gets to do it, unless that third person at the end of Return is supposed to be Qui-Gonn (and it isn't...)

So, yeah, good fight choreography.

And who says the spaceship was still generating its own gravity?
(equally full of holes, I know; don't try to make it scientific, it just won't work)