What makes a great photograph is when it can tell a story, photographs like these usually are shot during the time of tragic events such as the aftermath of the Vietnam War bombings or the deaths of unarmed people like in the Sterling case #Blacklivesmatter. These photographs come with aspects like lighting, focal point, emotion.etc, they keep viewers wondering, guessing, interested and overall send a message.
The photo should be able to capture the experience and the viewer should be able to walk away with something learned
I agree that if a photograph tells an important story it is a great photograph.