medjai:

Aggression: Be aggressive against attackers.

Strength: This is the most important physical attribute of a defender. You need to be strong because defending involves a lot of pushing, running, harrying, and what have you.

Heading: Good aerial ability to deal with crosses and aerial threats.

Bravery: That is, when getting into tackles, you shouldn't be afraid.

Positioning: This is one of the most important skills of a defender. The ability to be at the right place at the right time. It also means being able to read the game.

Creativity/ flair/attacking instinct: Back in the 90's, this won't be considered an essential attribute of a defender but now the game's change. Some teams, Barcelona, for example, like to build their attack from the defence. You hardly see them clear out the ball, rather, they turn defence into attack. The defender sometimes is required to help bolster the team's attack especially when the team is desperately in need of a goal. Countless times, we've seen Pique, Vidic, etc, score crucial goals for clubs. Truth is, centrebacks nowadays must have some attacking instinct in them.
It doesn't have to be only at set pieces that they go up front. Everybody, defenders inclusive, must contribute to the team's attack.

Tackling: Be good with your tackles. Know when to go hard or otherwise.