In soccer, if you think of a defender you think toughness, someone who's a figher, someone who's tireless, and someone who will do anything to keep the ball out of their own net. Someone like Barcelona's Carlos Puyol for instance, who usually leaves the game with a bandage on his head.
But in today's game, while all those qualities are necessary, defenders also have to be able to get into the attack and serve in crosses and play the ball out of the back to start an attack. It's rare these days to see a defender who doesn't have good touch on the ball and can make a perfect forty yard pass across the field. That's the modern defender, someone who tackles hard but has a soft first touch on the ball. The modern defender is some like Barcelona's Dani Alves or Chelsea's Ashely Cole, both defenders who are tough and tackle hard but can also play give and goes and whip crosses into the box.
Here's a list of attributes all defenders should possess:
High work rate (tireless).
Gets stuck in. Tough tackler of the ball.
Can pick out teammates with a long ball.
Wins high balls and doesn't let the ball bounce when they're trying to control the ball - first to the ball.
Reads the game well so they can stop attacks before they start.
Knows strengths and weaknesses (plays within themselves).
Crosses the ball well.
Serves in good long balls (driven passes) to the forwards to hold on to.
Communication with other defenders to hold line and keep the other team offside.
Knows when to make a professional foul - stop an attack by fouling the attacker but not so much to earn a red card.
And while it's true that modern defenders are now a big part of the attack and getting down the line to cross the ball, they can't forget about their defensive duties. This means a defender out wide, say the right or left, has to be in very good shape and ready to make those forty or sixty yard runs if the ball is lost and the other team is on the counter attack. It also means picking and chosing the times they go forward, making sure there's someone who can cover for them if they do get deep into the offensive third.
Watch great tackles from the pros and defenders scoring goals: