It's not sharpness.

I will say two things: 1. the subject; 2. how it is composed or framed. It can be a beautiful scene or something meaningful that grabs the viewer's emotions. A great photo will generally fall into one of two categories: 1. nature; 2. a human emotional situation. I saw two shots decades ago that still stick in my mind. I think both were taken by Gene Smith. One shows a little boy and girl, from behind, walking through bushes into the light. The other shows a Japanese woman holding her son, deformed by Thalidomide.