Well, I was never a defender, but as a forward, I faced a lot of 'em. To me, the best defenders are ones who position themselves well. I would have to say that positioning is probably the #1 thing. Being strong in the air is another.
I'm a defender myself and I feel that having a good attacking sense makes a good defender. I play outside D and i often dribble up or run up with the attack and over lap with the mids to create good combinations and my coach loves it haha. Roberto Carlos does an excellent job of mixing himself up with the attack and gettin strikes on goal in a lot, i think he is a good example of what i'm talking about.
Center Fulls should never be marking up. Ever. Their job is to clean up the mess that their outside fulls make, and if they have to mark up at the same time, they won't do a very good job. Any balls played over the heads of the fullbacks should be claimed by the center full. They should always keep depth in the backfield. If they get caught flat, fastbreaks will occur. I have to know this, otherwise I get scored on! Not fun!
this is exactly what i was going to say, i also think a defender should be vocal, get the talk starting from the back
this should probably apply to all players but a good central back should know when to take a risk and not to take a risk. One of our central backs tried to bicycle a through ball in the air . he missed and almost cost a goal. needless to say our coach had a good talk with him after that.
Myself as a forward for school, I play Defense, or Defensive Midfield for club, I'd say Patience, and Vision, and first touch. Positioning also another key part.
As someone who has played both marking back and center back a good defender possesses the ability to position himself/herself well, and has exceptional field vision. A good marking back has speed, power, a pressence in the air, can position themselves well, and keeps his legs closed. Closing speed doesn't hurt, but the striker should never get away anyway. A center back should not have to mark anyone unless there's a free or corner kick coming in. A center back should have the small skill sets as the marking back, plus strong leadership.
Positioning is 90% of defending and the other 10 is making good decisions and having speed to make up for mistakes you might make. I also think that a defender that sets himself apart from the rest is one that can get up into the attacking third effectively and be an unmarked threat, I.E. Roberto Carlos as a defender you have to be always in position though so playing that style of a defender can take alot of communication and a hell of a lot of running.
I have played Center D and Outside. I think things that helped me the most was being able to contian attackers, not getting to aggressive and diving in to early. you also have to be able to support your team in the attack and be able to read the game. Good defenders can see what the other team is building. also communtication is very important, aspecialy if you are a sweeper. you have to talk to your team and your out side defenders.
Several people have high-lighted on being vocal and knowing your position, but in my experience a good sweeper has the best ball control on the team. I can't think of too many reasons why at the moment, but it should make sense. The last line of defense that is still a field player should be able to control the ball better than most players on the team.
you have to be able to force the forwards to where you want them. if you dont do this they will control you. Also you need to be physical. You must challenge the ball whenever possible. you must also be able to contain the other player and wait for him/her to make a move first or you WILL get beat.