Rafael Nadal had a solid 2009 season, but injuries slowed him down. That leaves 2010 as a big year for Rafa. It's a chance for Rafa to show that he's still one of if not the best.
It's also a chance for Rafa to show that he can stay healthy. With his all-out running style of play, Rafa will always have a greater chance of getting injured compared to other players. It's going to be really important for him to choose his schedule wisely this year.
He's still clearly the best clay-courter in the game. However, I think it's important for him not to over-play in the clay court season. The long grind of the clay court season wears him down, especially when he heads to the U.S. hard court season.
To me Rafa could easily gain the top spot back, but a lot will depend on his health. One edge Roger Federer has on him, is that Roger knows how to work his schedule and he rarely gets injured.
Rafa is still young and hopefully is learning more and more how to work his schedule and take care of himself. If he learns that it will be great for the game, because whether you're a fan of Rafa's or not, everyone should want to see him at his best. He plays with the heart of a champion and he's very good for the game of tennis.
I think this year Rafa will bounce back from his injuries at the end of last year and will have a big season.