A great defender will be very athletic, fast, have good anticipation etc.... the same skill set that any position benefits from. But most teams don't have the depth to have players like that play defense. Typical trickle down has most teams with the very best players and atheletes playing midfield. I would also agree with previous posts that it it much safer to hide a weak player on attack. Your team can control the action run through an attacker, but an opposing team will target a weak defender and exploit her. But some of the very best players will be attack as well. Most defenders on the college level will be converted midfielders. The hardest part of it all is convincing the athletic and skilled kids to even want to play defense, especially when a team has plenty of talent depth.