Composition 9/11

Balance
The buildings on both sides of the focal point, the smoke, are nicely balanced to the eye. Both sides seem even, and the smoke seems to be both in the foreground and background. 

Avoiding Merger
The stark contrast from the firefighter and the flag make the firefighter stand out. If the firefighter was wearing red he would have blended a bit to much.

Framing
The buildings on the side and the building below the focal point of the picture, the explosion  dust,  make the dust appear out of place. Without that, the other things below the dust would make a foreground distraction.

Lines
The Twin Towers make complementary lines against the background of the blue sky. The foreground of the bridge and the buildings add more interest to the Towers without confusing the viewer of the main focal point.

Simplicity
The man in the front is obviously the focal point because the background is blurred. Although you can see the clock in the background and also the police car, the man is right in the foreground and the point of attention.

Rule of Thirds
The explosion in the upper right quarter is the focal point of this photo and it just so happens that it is also in line with the rule of thirds. The explosion seems so out of lace that it is not even necessary to make it follow the rule of thirds, but it is good that it does follow the rule.