I have this peculiar idea that I cannot really put in to words, past what I am about to write.
I think I like photos (which is the same as saying "I think good photos are...") which are what they are trying to be as hard as they can.
It's some sort of authenticity thing? Weston's pepper is an eroticized green pepper, as hard as it knows how to be. Powerful photojournalism/documentary photos genuinely are photos of something powerful and perhaps important. A good portrait is trying as hard as it can to be a window into the subject's soul.
Most photos aren't trying very hard to be anything, except possibly a copy of another photo.
Anyways, that's several days of noodling there, and it's not a lot.