A Great photo to me is one that cannot be exactly copied by anyone else. Unique to the photographer. Something that makes you say, " Wow, I wish I was there to see that ". One that you know has taken the photographer a lot of time, skill and effort to achieve. Then there is the luck factor. Right place, right time, right camera setting for that " great " shot. Greatness as well as beauty is definately in the eye of the beholder. As has been already stated, One mans treasure is another mans junk. To be truely Great though, I feel that the image should be straight from the camera, before any PP. In my opinion that is true greatness.