A Better, Darker, Meaner Darth Vader
The Darth Vader that we saw at the end of the Star Wars prequel trilogy was in a much different place. Princess Leia had died and with that so had Vader's thoughts of being a father. He was no longer fully human. The Darth Vader that we see in Rogue One: A Star Wars Story is a completely different animal. In his big lightsaber wielding scene, in which he goes after the plans of the Death Star, it is as if a wild animal has been unleashed. He literally destroys everybody in the room. Multiple laser beams are coming at him and he deflects every one with ease. Aside from this, every scene he is in is terse and fraught with tension. Simply put, Darth Vader is not messing around. So well done is Rogue One: A Star Wars Story that we almost feel compelled to watch Episode 4 immediately just to see how the portrayal of this character, across many years and films, connect. Sadly, the prequel trilogy never really comes as close as Rogue One to showing us this much complexity of the man in black. Granted, he doesn't have nearly as much screentime, but that in itself is a problem.
Rogue One looks more like A New Hope
This might be scandalous to say but the retro-look of Rogue One is probably 50% of why it is being received so well. Episodes 4, 5 and 6 have been with us forever. We have a certain level of expectation when we see them. (This is why George Lucas's constant tinkering with them bothers so many fans, it seems.) So when we see a film that is supposed to be part of these long gestating film's lore, we want that film to dovetail into our expectations. This was probably where the prequel trilogy is most wanting. Rogue One, for a big budget, popcorn, Christmas film is wonderfully restrained. It is rich with a look that recalls the past films (namely Episode 4), but it doesn't always reside there. We are taken to places on screen that we have never seen before in the Star Wars Universe. As a result that retro-look, like Rogue One itself, is free to shift, move and become something totally different, yet remain comfortably old school in certain parts of its aesthetic. Sadly, the Star Wars Prequel trilogy went in such a different direction, it appeared at times as if the subject matter was serving the look of the films instead of the other way around.