I am using this system to rate photos along two dimensions: Aesthetics and Originality.
Aesthetics
1 = Ugly; 7 = Beautiful.
Reasons for a rating closer to 7:
looks good
attracts/holds attention
interesting composition
great use of color
(if photojournalism) drama, humor, impact
(if sports) peak moment, struggle of athlete
Originality
1 = Hackneyed/Boring; 7 = Amazingly Unusual.
It is original if you haven't seen anything like it before or if you wouldn't have thought of this approach to the subject.