A strength of this composition is that the photo is in black and white, which diverts viewers from focusing on the different colors and more on the content of the image alone. Another strength would be that the subject of the photo is just a little off-center, so you’re able to see the other people blurred out on the left side of the image; this draws more immediate attention to the in-focus object.
The artist’s choice of depth contributes to the composition because the photo focuses on the veteran rather than any background objects. Blurring the background attracts the viewer to the main subject of the photo- the veteran and his flag. This short depth of field demonstrates the importance of the subject only.
Diane Arbus: http://www.artnet.com/artists/diane-arbus/
Good observation of the subject and depth. You can also mention light and perspective in this example and the contrast between the sky and the subject.