Turn offs:
Inartful nudity or porn
Snapshots parading around as photographs
Color, when B&W would be exquisite
B&W, when color would have produced a superior result
The average pregnancy picture. However some I have seen were incredibly well done.

Turn ons:
Any well done photograph, even if it is only suitable as a textbook illustration, etc. Photography is an art and a science and good stuff is good stuff.