He plays a ridiculous low-unforced error, defensive style of tennis that is somehow essentially offensive in nature. He's got low unforced error counts, great depth and penetration on his shots, and controls a high percentage of points. It's crazy. He's the worst type of opponent you could face, rarely will ever beat himself, has court coverage abilities that make it hard to hit through him or dictate against him, and he's constantly able to move the ball and dictate the point himself - all while making minimal unforced errors in a match.
I've loved Federer's shotmaking, Nadal's clay court game, but Novak really has the whole package. He's a phenomenal tennis player.