I'd say the keyword is balance. He is one of the more complete players ever, he has virtually no weaknesses, except maybe in some specific cases at the net under pressure. He was always a good defender, in Rafa's style, but now he regularly manages to get the upper hand by being agressive and not relying as much on the other player mistakes. Even against players like Federer he's agressive. He'll never be a pure offensive player like Roger, but he doesn't make as much errors so it's a fair deal. He has an excellent physical condition, so he's a nightmare to play in long matches because he'll always have ups after downs, if he has any. He's not the more powerful or muscular guy out there but he has excellent timing and eye so his strokes are always efficient. He managed to build a strong confidence after a lot of difficult wins over Rafa, Roger and Murray, so nobody on tour scares him.
I know his supporters aren't going to like this but, to me, his current domination is also due to the fact his main opponents aren't as competitive anymore, and with the recent evolution of the game, young players take more time to mature.
IMO he's as good as Roger and Rafa.