GOOD: Well composed, exposed, and interesting. Subject matter recognizable, and represents photography at an advanced level.
GREAT: Well composed, exposed, and unusual. Subject matter draws attention, framed in a manner that is not commonly seen. Causes the viewer to examine closely the photograph, image seems to reflect purpose, and captured at a special moment.
EXCEPTIONAL: Composition and exposure used to create an unexpected emotion and feeling with a haunting residual impact. Has an intuitive story that may differ from observer to observer, but lingers in the memory and commonly described as unforgettable.
These are not textbook definitions so don't look for a resource footnote. This is purely my way of putting into words, a way of distinguishing levels of photography. Good and Great are shades of the same thing, one being more creative and memorable, while exceptional is on a different plateau, where rules don't apply,