A great photograph should grab your attention by capturing your interest, be well composed, with an emphasis on light, color, design and composition that is aesthetically and technically without any distracting visible flaws. It should tell a story that is easy to understand at a quick glance, evoking a feeling that anyone will be able to recognize immediately upon looking at it, and should capture a moment in time that cannot be replicated.
A great photograph should be so perfect that nothing seems as if it should be added or taken away.
Certainly a clean background that does not distract can help a lot!