I am getting carried away!
What does make a great painting?
I believe it is the space for the viewer, that is left open in the painting, which is how a great painting begins. Second, your memories are triggered by a painting and you become the spark that completes the greatness.
Why are some paintings so universally appreciated? Is it that the more common the subject, like Mona Lisa's smile, the more everyone can get involved?
Whatever it is, be sure you leave room for the viewer, because you are the viewer too!