The RotJ fight, as a fight in itself, makes me gnash my teeth.
In a bad way? Why, sir?
Keep in mind that I'm talking purely from a "fighter's perspective", in a way. As a dramatic element, the fight is beautiful. But the way it goes makes me roll my eyes.
At first, the fight's fine. It's the very second that Luke ignites his saber in rage that the fight tumbles down "The Hill of BS" (tm). Vader provokes Luke after searching his mind and Luke comes at him through ambush; this is fine. However, Luke falls into using one tactic: swing home. Literally swinging his lightsaber from side to side like a deranged chimp, Vader is somehow unable to do nothing more than deflect Luke's lightsaber and back away. More deranged horizontal swinging and we're left with Vader somehow tripping like a scared female in a B-grade horror movie and letting Luke hammer at him for a few moments before Luke decides, "Hey, I'll cut his hand off. Won't that be a kicker?!"
If Vader had any semblance of skill, it would've been all too easy for him to overcome an enraged opponent whose skill becomes handicapped when angry. Every wide, horizontal swing presents an opportunity for either a thrust or vertical strike. Luke ends up turned sideways, bouncing his saber into other objects often, giving an easy shot at that entire side of his body and positioning his weapon so that he would've been unable to do anything about it. Luke was more open than an all you can eat buffet on its opening day and yet he somehow still won.
Any sort of perceived "power" that Luke might've had during the fight had nothing to do with it and from my own opinion, I think it proves that Vader really didn't want to kill Luke. Otherwise, he would have. Luke presented him with far too many opportunities, and if we're led to believe that Vader gave it his all in both mind-set and ability, then it really is a badly written, or perhaps badly performed, fight.
OHHHHHH.
Yeah, I agree that Vader falling down was COMPLETELY unbelievable. He had no reason to do so. Although if you account for Luke's overwhelming aura in his hatred, I think anyone would have fallen. But I do know what you mean. In the film it did look like he leaned back against the rail for NO GODDAM REASON. Blame the man in the suit (David Prowse - not a professional actor). It's really the actor's fault. And when Luke kept hitting his stationary saber - Vader wasn't even fighting back! That, admittedly, sucked a little bit (unless you rationalize it by saying that Luke's anger was overwhelming them both). However, the whole scene, I thought, was cool otherwise.