Nadal was absolutely formidable on clay this year, far superior to his 2011 clay-court form. In 2011, Nadal was just doing enough to win matches on clay (unconvincingly), and got beaten twice by Djokovic in clay finals. It wasn't until the latter stages of the 2011 French Open that he started playing well, and Djokovic was taken out by Federer (so all the better).
In 2012, apart from the blue clay fiasco in Madrid, Nadal was absolutely dominant. He won Monte Carlo without dropping a set, won Barcelona without dropping a set, won Rome without dropping a set, got to the French Open final without dropping a set, and then beat Djokovic in 4 sets to win a 7th French Open and thwart the Djokovic Slam.