Exposure
Lighting
Contrast
Focus
Sharpness
Depth of field
Position and strength of main subject
Rule of thirds (following it or breaking it)
Number and arrangement of supporting objects:
Leading lines, diagonals, S curves, etc.
Cropping & Orientation
Portrait, landscape, panoramic, square, etc.
Choice of backgrounds, colors, and saturation
Distractions:
edge mergers, subject crowding, messy background, etc.
Does it tell a story?
Does it elicit emotion?
Would a color photo be more impactful if chaged to black & white?
Would you want to hang it on your wall or share it with others?