Accuracy. He just hardly ever makes mistakes, and keeps testing his opponent move after move.
Magnus Carlsen
Accuracy combined with the will to win. Carlsen wants to win so much so that he'll play 30 extra accurate moves in a level position to see of his opponent blunders.
Some other guys like Nakamura have that same will to win, but they don't have his accuracy.
What does carlsen's game have that has made him almost unbeatable?