He's too content with waiting for the ball to be played to his feet. It's not just his drive, so to speak, but it's the things he does, or rather doesn't do, that's the issue.
He doesn't make any off-ball runs that provoke any kind of through-balls or lofted balls to him. (See Sterling for the opposite of this). Martial is excellent in front of goal, so can someone tell me why he isn't making the kind of runs that make best use of that? When you have Pogba behind you, who btw can spot and pick any pass, you know you're not doing it right if even he can't find you.
He always wants the ball at his feet with a one-on-one opportunity to go at his full-back. Unless you are playing for Man City, this isn't going to happen a lot at United. Tactical passengers are no good for us. We need as much movement off the ball as on it.
He's very sloppy with his passing and general play. Too hard, too soft, wrong pass angle, wrong passing option. Even on his best days there's a problem with his passing consistency. He's superbly talented, and yet he never plays to a standard that is befitting of that talent, and therefore, presents a problem for a team trying to compete for honours.