Brwned has hit the nail on the head. Forget the stats, possession and what not, he poses a threat. It's something that we lack on most of our other players. Yes Young can run up and down all day long, run at players and provide good width, and he has done really well, but his acts in the final third are nowhere near to what Di Maria is capable of. We have to work towards getting that out of him consistently. Even when Di Maria fecks up in the end, it is only due to his brainfart or a wrong decision, the opposition usually have no clue how to deal with him. And those "brainfarts" or things that look bad in hindsight come with every ambitious and creative player. He tries to make things happen with every touch, and has the technical ability on the ball to execute that, better than anyone else we have. Plus, his best attribute by far, is the world class vision that he has. It's something you can't train, and even when he's not hitting the passes perfectly, the runs and movements he picks out wouldn't be picked out by someone like Young in a whole season, no matter how hard he works. He adds tremendous amount of unpredictability in our game, and he can only get better.
And of course, he has genuine scary pace, which is simply a big weapon to hold.