I would have liked to see more 70's style sci-fi art direction. The PT was too modern, too much like Star Trek TNG.
I would have liked to see clothes made from 70's patterns, people having 70's haircuts and environments and space ships more inspired by Ralph McQuarrie, i.e. the man who designed the look for the original trilogy. I want the Star Wars feel, but on a more grandiose scale.
For instance, I would have liked to see this scene, as drawn by Ralph McQuarrie.
GL claimed that he cast Hamill, Fisher and Ford as a team, because the chemistry between them was just right. That is also something that is lacking from the prequels, that he did not do.
What GL should have done more than anything else would have been to let other people (SMART people) influence how the movies were designed and made. For instance, he needed a "Marcia Lucas" to tell him when his ideas were bollocks.