Does genre really matter?

I got in trouble with a reviewer once for describing a new adult book as young adult. (Don't miss Alexandra Lanc's visit to my blog next month where she explains the difference.) Another time I reviewed a children's adventure that was really young adult. Then there are mysteries that are actually horror, romances that read more like drama, Christian fiction with more of a vaguely spiritual fantasy feel. (Don't forget edgy Christian fiction too). A friend wanted to write a query letter and ask...