Nadal beat Djokovic for the title, concluding two days' play on 6/11/12. Sports fans care about the results, but real fans analyze the players and plays.
Nadal is suited for red clay, and a sunny, dry day is his best friend. Under such conditions the tennis ball rotates the most and bounces the highest, giving him plenty of time to get to and hit it and his opposition trouble returning it. If 6/10/12 were a normal, clear summer day, Nadal would have won by 3:0.
The rain helped Djokovic a lot. The ball got wet, slippery, heavy, and hard to spin. It also made Djokovic's flat hits sink faster. It was unusual but explainable for Nadal to lose 8 games straight during that time.
Going forwards, if next year's final is on a clear day and both are healthy, Nadal will win again.
To the other surfaces it is a different story, and need to include Federer for comparisons.
Federer, Nadal, and Djokovic all have excellent techniques and footwork, all are fast and strong. Without Nadal, Federer would have won 20 GS or more, ending any discussion about GOAT. Without Djokovic, Nadal would have caught up with Sampras by now, seriously challenging Federer's totals. Federer's style has precedents, even though he masters it the best. The emergence of Nadal and Djokovic is more "accidental", an example of sports and hence history being influenced by chance.
The weapon of Nadal is his running ability and big top spin from a left-hander. The last point is a killer to a single-handed backhand right-hander like Federer. Because his style matches so well to Federer's, it is natural that Nadal is Federer's nemesis. As long as Nadal keeps spinning to Federer's backhand, Federer will eventually make an error. Federer can't use his big forehand alone because Nadal retrieves too many shots; once Nadal returns the ball to Federer's backhand, we get the same thing all over again.
This situation is reversed when Nadal plays Djokovic. Djokovic is taller and has longer arms and stretch that handle high balls better. More importantly, Djokovic has the most balanced forehand and backhand in the history of tennis (and the best hair). Like Nadal against Federer, Djokovic has a simple plan. Serve to Nadal's backhand, then attack his forehand open court right away. When in defense, hit to Nadal's backhand, then turn to offense. One sees this formula time and time again when they play. Why can't anybody else do it against Nadal, as there is no secret? Because only Djokovic can attack from anywhere in the court with his balanced forehand and backhand.
How can Nadal counter this strategy? On clay Djokovic's strikes are less lethal. Nadal actually anticipated serves to the backhand side very well during the French Open final such that Djokovic had to mix it up by serving to his forehand more. Surprising in a sense that although Nadal didn't give Djokovic as many chances as before, Nadal didn't have an advantage there either by returning serves from the backhand side. Serving to Nadal's forehand too much seemed to be overthinking on Djokovic' part. Overall Nadal can improve his backhand or lob a few service returns to buy time. Or run around to use his forehand, this option will be more tiring and only works when Nadal is >100%. Nadal should also use backhand slices more, deep and low. They still do not guarantee success, but Nadal can hope that by doing so he wins all the big points and the match. He also needs to serve well to deny Djokovic opportunities of counterattacks.
Djokovic's style or strategy does not work as well against Federer at his prime. Federer's single-handed backhand has too many variations. When Djokovic attacks his backhand, he can slice more naturally to deep corners or the middle to make Djokovic use more energy to lift the ball to induce errors, while at the same time allowing Federer time to recover. Djokovic doesn't have the top-spinning ability of Nadal, so he can't exploit Federer's weakness as much. The recent success of Djokovic over Federer is not due to any strategic superiority but because Federer is getting too old and slow.
Now, where do the upcoming players stand, and how will the playing style evolve? Tennis techniques haven't changed much over 30 years, with Nadal's top spinning being perhaps the exception. Players are simply stronger with better rackets to execute better. It can be imagined that Djokovic's style will be the easiest to imitate by young players. Balanced backhand and forehand, top spin and flat hit, fast and precise serves. On the other hand, it becomes boring when everybody plays the same style. This would have already been the case if not for some of the greatest players ever playing today. Look at women's tennis today, minus the beauty contest. Once these three musketeers retire, what will be there for a surprise?
Personally, I wish there will be another Federer with a stronger backhand.