I think that you're right, Rob C - asking artists what art is isn't necessarily the best way to go. But I also don't know that the general public (by which I suppose we mean something like, the majority of people who don't have specialist training in fine art) have got a good answer, either. (Would we consult a non-specialist public if we wanted a definition of the term 'neurology'? It probably wouldn't be our first choice.)
As far as I can tell, there's no clear, generally agreed definition of 'fine art'. What there is, is a lot of discussion about what it is. In fact, you could say that that's one of the defining characteristics of fine art - that it repeatedly raises the question of what art is. Art can raise lots of other questions, too, but that question keeps cropping up. And it crops up not just among artists, but among writers, critics, curators, dealers, theorists and historians too. As a result of the arguments that take place, areas of consensus develop - but they continue to be contested. Thus, we could say that art's a pretty unstable thing, and not just one thing.
I think a way to proceed is suggested in one of your earlier comments about the way photography can have different roles in different contexts, and by the example of the same photograph appearing in different contexts (fridge door, gallery wall etc.). The question stops being 'what is fine art photography' and becomes 'under what circumstances might a photograph become fine art'? The answer then will be primarily concerned with context - possibly a combination of intention, location, and reception. Is the photograph intended to be engaged as a work of fine art? Is it presented in a form that fine art is or could be presented? Is it received, treated and possibly exchanged as a work of fine art?
This last position leaves things very open still, but I think it's the way the art world works. The greater the authority of all three constituents - by which I mean, the more readily they are legitimised in the current art world - the clearer the status of the photograph as a work of art.
In short, the art world (artists, curators, galleries, critics, collectors and so on) decides what is important art. And its decisions can be underwritten by all kinds of motives, some of which are explicit, some not; some justified, some not. What we can do, is to continue to ask on what grounds such decisions are justified.
Well, that's the way it looks to me.