The fact that the lightsaber battles changed in the prequels has nothing to do with the advencement in film-making technology - Lucas stated in the audio-commentary of Ep 1 that we were now seeing Jedi knights in their prime - fully active, young and ready to kick proper ass unlike the crippled old men (Vader/Ben) and young trainee (luke) from the original trilogy.
The T800 in T4 shouldnt do anything more than he did in the previous films. He is supposed to be stiff and limited in movement cos he has NO skin or muscle. He walks that way cos his feet are obviously not supported my tissue!!!
What Lucas said is indeed true but he is also the same guy who kept on touching up his OT so that there is less disparity in terms of visual and technology between the two trilogy.
The fact remains that back then, there wasn't the cgi or choreography to make the movie the way he wanted it and the prequel is really his way of finally doing a star wars movie the way he always wanted it.
In the OT disc 4, he mentioned that in a new hope, the reason for Vader and Obi Wan holding the lightsabers with two hands was that he wanted to give the impression that the lightsaber were so powerful that you need to grip it with two hands to be able to control it.
But in ESB and ROTJ, the fights became more fluid with one hand holding lightsabers, with might air twirling and the whole lightsaber swinging thing because by then he wanted the fight to be more fluid and more fancy to show the mastery of the lighsaber by a Jedi. Of course he took that even further with the prequel.
Yes, one could indeed say Obi is frail but Christopher Lee and the Emperor is frail too in the prequels but they sure are flying around with their lightsabers ain't they?
I can't believe the excuse you gave for the robotic movement is because no tissue and muscle? It's a robot for god's sake. The pistons and the gears, or whatever that's inside that metal shell that's making the robot move. The tissue or whatever muscle covering got nothing to do with how the robot moves.
It's just simple limitation with technology back then. They had to used a puppet for the T800 that's why it moves like a puppet. Arnie is human not matter how stiff his acting was, hence he still moves like a human.
But with CGI, the filmmaker can now make the T800 movement smoother and more human like (look how much more smoother and human like the TX moves in T3 cause that was fully CGI) instead of the robotic puppet from the 80s.