My observation is that, Rome and Madrid had no sun light on the court, and we all know that Nadal plays better on the high heat because the ball bounces higher. (Why Madrid didn't have Nadal playing on a direct sunlight, which favors their player... I don't know. The light was just going away when they were starting.) Notice the Indian Wells final was overcast too. The Miami final did have light, but now Djokovic is to a point where we can say he is playing a better hard court game than Nadal. If both meet at RG on a sunny day, I'd still favor Nadal though. Example is Nadal vs. Gasquet in Rome: they played on the high heat, and the ball was kicking too high for Gasquet to display the same game he displayed agaist Federer.
As for the improvement of Djokovic's game, I think his equipment change had to do a lot with it. He obviously wouldn't admit it, because he is under contract. There might be some customization done, aside from just changing to a black body racket, and that customization just nailed it: maybe he was missing that for a long time. When he played with the Wilson the last year he was with it, his results and game weren't bad at all. But we all know how abusive he was to his equipment once he started playing with the white Head. There are changes to his game as it's been noted already, but also notice his swings are not entirely different; instead they are almost identical to the swings he had before when he didn't play that well. With the black racket he hits with more ease too, which means probably the setup or the custom setup is more powerful... and add to that the custom weight balancing, and it seems the missing pieces of his game are in place now. To me it's similar to what Davydenko went through some years ago, when his game skyrocketed even further. It was his equipment change to a more powerful racket in Miami, and his game started talking on all pistons.... And he went on to win the tour finals with some brilliant display of tennis.
I also agree that Djokovic's breathing is better, but that might also be due to him using less effort on swinging... and less effort on his swings probably means his racket might be more powerful than before.