One of the things which changed most between the trilogies is the idea of the Jedi. During the original trilogy, Luke's costume in RotJ was what a Jedi wore. The Jedi were more mysterious, and more subtle. Creeping around with hoods up, using mind tricks, and so on. For Episode I, Lucas intended to continue with this. Here is some concept art of what the Jedi were going to look like: Then Lucas decided that he wanted the Jedi to be instantly recognisable to viewers, and so made them look like Ben Kenobi in the original trilogy. The effect of this is to make old Ben look even more ridiculous in A New Hope; even though he's supposed to be hiding from the empire, he changes only his first name, and still wears his Jedi robes for twenty years. The Jedi in the prequels are also conceptually different. They are more similar to (superpowered) warrior monks, whereas in the OT they were sly wizards and then swashbucklers. In the OT the Jedi were also uber-confident - note Luke and Ben's supreme calmness even facing certain death, and even telling their enemies of their own imminent demise. In the prequels, the Jedi are perhaps more human in their emotions, showing anger, surprise and panic.