I've done both erotic "fine art" nudes and porn and it seems to me that a big part of difference is that with porn it's explicit, you see everything (the surfer wants closeup pics of a hairy pussy, that's what he gets), whereas something might show nearly as much nudity, but not be considered porny if it's about the whole model, not about focusing on her hairy pussy, giant tits, webbed feet, whatever. A site like Met Art (
http://www.met-art.com/) usually falls on the tasteful side of the line because they take that approach.