Rule of Thirds

The Towers are on the right 1/3 side of the picture, and the smoke is on the top right, and gives the picture enough room for the plane to be in the middle.

Framing 

The broken wall works as a frame for the firefighter, and makes us focus on him/her.

Lines

The vertical lines makes it easy for us to focus on the falling man. Although the lines are not pointing towards him, the man is in the middle of the lines and it makes it easier to focus on him.

Simplicity

This picture is simple because it only shows the main point of the picture and nothing else. (except the tree)

Balance

There a balance of the the towers and the mourning people on the ground

No mergers

The pictures show us what not to do, like putting in the blue building that intersects the towers.