The middle linebacker often defines his defense. The great NFL defenses of this generation have taken on the personality of these players, like Ray Lewis with the Ravens, Derrick Brooks with the Bucs, Harry Carson with the Giants and Mike Singletary with the Bears.
Besides possessing great instincts, a middle linebacker is counted on to captain the defense and to be a leader in the locker room and on the field. Speed and quickness are great, but a nose for the football and an extensive knowledge of gap control are both key.
In the 4-3 defense, middle linebackers need to be "tackle-to-tackle" run stuffers with the ability to get off blocks and make the play at the line of scrimmage. Successful middle linebackers at any level have defensive lines that protect them by not allowing blockers to get to their feet, keeping them free to run to the football.
Middle linebackers do not necessarily have to be great athletes, but many are productive because they are tough, smart, short-range guys that are physical and set the tone for the defense.
In the 3-4 defense, the inside linebackers must be the same type of player, but his needs change and he only has to play half of the field a lot of the time. He also has to have more range and the ability to play in space since he will be asked to play in pass coverage.