There are many reasons why the PT and the OT look and feel different, but mostly I'm going to try to focus on the cinematic, not in-universe, reasons for the differences.

Firstly, the bulk of the Original Trilogy took place in space, or at least the blackness of space was ever-present in most scenes. This makes the mood of the film darker and more oppressive, which is perfect for symbolizing the oppression of the Empire on the rest of the galaxy. However, in the Prequel trilogy there are not many scenes which take place in space or have it anywhere near, most the films take place on planets. The planets are brighter, the scenes themselves are more colorful, and more vivid than those found in the OT.

This vividness gives a lighter, more open, feel to the films. While they were highly critisized for not maintaining the same feel of the older films, I don't see it as a bad thing. For in the Prequel era, things are not as oppressive as they become in the OT era. The empire has not yet taken over, and generally things are going pretty good, no one has yet felt the sting of the Emperor, and the strain he puts on the galaxy.

Now this may sound like an in universe explanation, but cinematically it was exactly what George Lucas needed to do. The lighter, brighter, more vivid style is what he needed to show that the times were different before the Empire, and much better. He needed to visually show it because he couldn't really take the time to explain that times were better, he had to tell Anakin's story. To tell the story he needed--and wanted--to tell with the prequels, he had to establish quickly (which with film the best way to do that is visually without explaining it through dialogue) the different time period, a time where the oppression did not exist.

I believe this is the reason why Revenge of the Sith is being harolded as a return to form, visually, and why so many critics said it felt and looked like the OT. The planets were darker, and not as vibrant in color. Even Felucia's colorful flowers were dulled and saturated. While there is hardly any space throughout the bulk of the film, the last scene with Darth Vader took place in a familiar setting...space, signifying that the oppression of the Empire has begun.

The last thing I am going to say about this, and this will cause quite a back lash I am sure, is that I think, visually, The Phantom Menace is pretty close to the OT. Now here me out, quite a bit of scenes happen in space, much more than the other 2 PT films, and I never really realized that until after watching Revenge of the Sith, and watched Menace again. Give it a try, and look for those things.

--DLZ