He is fantastic defensively, he's a very natural defender and has a lot of tenacity & aggression. He has a great stand-tackle & ability to jockey a winger (the most important things for a full-back, in my opinion), combined with unbelievable speed. He's like a street footballer, minus the natural ability on the ball. He has a huge amount of work to do with regards to his crossing and dribbling - they should improve over time, but I doubt he'll ever be an Evra. Defense can be trained into you (to a certain extent - not everyone can practice to become Cafu), though I feel it is much easier to see a player's attacking potential from the start. His final ball is severely under-par & he isn't so technical in the final third, but he bombs down the wing and finds himself in those positions, at least.
I really like him & think he looks like a genuine 'Manchester United' player. Huge talent & he is already making himself one of the first names on the team-sheet, when fit.
Edit: The amount of people who think he has no pace
He is Valencia-quick, he's like a steam-train once he gets going. Don't be fooled by the massive ass.