Quality:
low quality
blurry
pixelated
noisy
out of focus
jpeg artifacts
Composition:
text
watermark
logo
signature
cropped
tilted
poorly composed
Style:
unrealistic
cartoon
anime
sketch
painting (unless you specifically want a painting style)
Other:
violence
gore
nudity (depending on platform restrictions)
multiple faces (unless you specifically want it)
Additionally, consider these for specific types of images:

Portraits:
bad anatomy
bad proportions
missing features (like eyes, nose)
extra limbs
mutated features
Nature Scenes:
pollution
litter
unnatural weather (unless for a specific effect)
Cityscapes:
abandoned buildings
traffic jams