There is no single quality in my opinion..if there were, it would not be long before we could all bottle it and use it. If I applied your question strictly to photography, I would suggest that the over riding emotion that is triggered in a great image is that of a human value expressed strongly. Whether this be fear, love, hate, excietment, anger etc. That expression does not only arise from images of people, but from landscapes, and objects. That ofcourse is a quality, but is not in of itself the differentiator of why two images with the same human values rank differently. Execution practically of the image through choices of light , serendipity (as keith mentions), and composition all serve to accentuate the impact of that value in a variety of ways. Think of every great image, and you will associate with it a human condition and value. It's greatness is measured by how well the photographer has brought the elements at hand to deliver that value. At least, that is how I see it. K