Had they immediately launched into the prequels after RotJ, and had Revenge of the Sith done around '90, obviously the CGI and technology would not have been what we got.
But I think there is a couple of more interesting areas that would have changed.
1) Lucas was tired of the franchise by RotJ, he probably would have had less direct input and allowed the various directors and writers more creative control of our hypothetical prequels. Which probably would have been a good thing.
2) We were still in the Cold War at that point. The idea of America being the only superpower and world policeman, and the apathy about that role that started in the '90s and kept building did not exist yet. I think the prequels would have drawn less parallels between the fall of Rome and the corruption of American democracy, and instead focused more on some sort of civil war scenario without Palpatine controlling everything, but instead taking advantage of the situation near the end.
The idea of one man controlling and manipulating everything was a very post-Cold War idea. I think we would have instead seen something more along the lines of two competing worldviews fighting for control of the Republic perhaps as thinly veiled Capitalist and Communist analogies. Which I think would have been more interesting than the Seperatists being the duped, easily manipulated opportunists without a shred of true ideology that we got in the Prequels.