I have three rules for knowing a great photo, all starting with the letter N: Nice, New, Nifficult.
Nice is the result of following traditional advice about composition, background, color, subject, lighting, and details in shadows and highlights. Nice is what you see repeatedly displayed in sleek men's magazines -- the same, monotonous poses of beautiful women. You've seen one, you've see all. Because nice is the product of education.
New is innovation in approach, such as shooting from an unexpected angle, from a tree for instance, or from an extremely low position, or presenting a subject never seen before, such as a transparent frog, or arranging the elements so that a strikingly new combination comes out. New is beyond education. It is the realm of discovery, and makes the viewer find excitement again in a drab world.
Nifficult is the mark of rare talent and makes the viewer, and even a professional photographer, wonder: "How did he do that?" Nifficult is the product of patience and unusual technique -- of spending days and nights in the forest, waiting for the elusive beast to spring into the frame of recognition, or inventing a gadget to bring a live human fetus into focus. The process of making a Nifficult photo is difficult to duplicate, if at all.
Many pictures featured in National Geographic are not simply nice, but usually new, and those that are made from a difficult journey or contraption or angle usually comes out on the front page. Steve McCurry's "Afghan Girl" is my favorite nifficult: a rare subject, her eyes luminous jewels, sneaked out by a frightened princess from the terrible landscape of war.