What makes a great photo?

"A little while ago, someone sent me an email and asked me whether I could tell him what made a photo great. I wrote back that I had been thinking about it (so far, so good) and that I was in the process of writing an entry about it (well...). In principle, it's probably the easiest question to ask. We know a great photo when we see one. But then describing what it is that makes it great is an entirely different matter. And, of course, different people pick different photos. So I thought it would be quite silly if I wrote an entry about this all on my own and pretended I was some kind of authority. Instead, I emailed as many friends, fellow bloggers, and photographers as possible -- looking through my 'contacts' -- and asked them instead, the idea being that, in the end, it is probably the sum of what every single person has to say that will answer the question 'What makes a photo great?' -- or maybe not (which would make it all the more interesting)."