All this is important but let's not lose sight of why most of us write in the first place. I think writers write for much the same reason as readers read. A reader picks up a book, and sticks with it, in order to escape to that world for a while. A writer writes in order to create that world.
Writers are generally imaginative people and we like to escape for a while into the worlds we created, too! Given the hundreds of hours it takes to write a novel, it had better be a world that captures our heart. Writing the book of your heart doesn't mean abandoning marketing concerns and the rest of it, but it does require some emotional investment—it requires heart.
So don't let anyone get away with asking, incredulously, You're writing about WHAT?