I agree with Thomas - it's contextual. If I'm shooting a purposeful project, I'm highly unlikely to take a shot of anything that doesn't fit the project. But if for example, I'm on vacation and traveling with my camera, then just about anything I come across that's graphically interesting is fair game. But even then, there's still an internal editing process as to what I will or won't photograph, because I'm looking for certain things. You just get in a certain head-space to look for certain kinds of things to the exclusion of others - abstracts, details, colors, textures, people, events, etc., and anything outside of that you either ignore or it breaks your train of thought. Today I might be looking to shoot color night photos, and tomorrow, I'm shooting people. Or it could be that I'm photographing a place, and then anything in that place that expresses the spirit of the place is fair game.