Maybe I heard it and didn't actually think it, but somehow it got wired into my head a long time ago that mainstream lit is ordinary people in ordinary circs. Genre is extraordinary people in ordinary circs (argument aside about extraordinary "ordinary" folks as opposed to powers or aliens, or other-than-humans) or ordinary folks in extraordinary circs (going to another world, discovering vampires, or time gates here), but the most difficult to pull off is extraordinary people in extraordinary circumstances. There is no anchor point, so the writer has to try to build one by implication.