For many of the genre readers I know, YA is also a way of identifying books (especially fantasy) that they want to read. Personally, I think this is because the generally shorter format of books classified YA necessarily eliminates epic Tolkien imitators (the "big bucks fantasy novel" or "extruded fantasy product") and favors more innovative works. Also, it is my belief that YA fantasy is much more likely to focus in on character, whether or not the novel follows the popular rite of passage plot, since I think one of the hallmarks of YA fiction is that it offers the reader characters with whom they can identify. Not that other fiction neglects this, but I think in YA fiction, it's more of a priority.
I don't think it's required to squeeze books into categories and then tie them there so they cannot escape. But I am curious as to what others see are common qualities of books that they consider Young Adult.