Re: previous discussion about how much the weather can affect the outcome:
Look at what Novak was doing to Nadal in the first set before the courts dried up. That's how much tennis is affected by little things. The king of clay almost got bagelled on a court he hadn't lost on in 8 years. The ball was staying so low, Rafa's forehand had no action, and Novak's flat shots were so penetrating. Some of his DTL forehands had to be picked up by Rafa off of his shoelaces. When the courts dried up, Rafa made it more competitive, but ultimately, despite some lapses, Novak was too in the zone.
I feel after such a slow start, it was always going to be an uphill battle for Nadal. Some tactical adjustments like hitting his forehand a little more up the line, and using the backhand slice down the line to Novak's backhand were fruitful and almost won him the 2nd set, but with so many errors off of the backhand side, and a relatively tame service day was always going to make it difficult. Novak was putting so much pressure with his returning, and pinned Rafa behind his backhand, who despite hitting it well as off late, just couldn't hit with enough consistency off of that side (at least he tried to hit through it as opposed to dropping it short and getting killed).
Nadal's forehand not firing (especially his hesitancy to pull the trigger on the inside out forehand) didn't help matters. Because it's clay, Rafa just wouldn't pull the trigger early because that's not how he plays on this surface (by contrast, he was willing to make that adjustment at the US Open and AO when they played). I can't blame him to be honest, and I don't think he should have necessarily pulled the trigger early, but ultimately, there were too many instances in the second set where he worked himself into a position to hit the winner and just didn't have the confidence to go for it.
It was obvious that he wasn't defending as well as he did against Novak in say, Rome of last year, and Novak wasn't giving him nearly as many unforced errors, so Rafa had to get more aggressive with the forehand. He wasn't playing good enough to do that, and his opponent was playing too good to give him many chances at an adjustment.
As far as Novak is concerned, it was the typical winning strategy against Rafa, and it often boils down to execution (he wasn't executing that well last year, and lost 3 in a row). He was much sharper this year, and won. Serve out wide on both sides, open up the court, and dominate from there. He makes it look quite simple actually, when he's playing that well.
When they inevitably play again on clay this year, Rafa needs to play more to the middle, give Novak less angles, and get that inside out forehand going. More importantly, he needs better placement on his serving. Every service game is going to be a struggle otherwise.