The first major feature that most YA stories have in common seems to be the age of the protagonists. Generally human characters will be teenagers for the majority of the story, and in stories where the main characters are other than human, the characters will either appear as teenagers to those around them, or show other character traits indicating they are not yet (but soon to be) adult members of their species. Not that every story about teenagers, or every coming-of-age story, is aimed at or enjoyed by that group, but there's a common thread there. Likewise it seems to be generally acknowledged that young persons prefer to read about protagonists that are the same age as them or maybe very slightly older, at least when the main story starts.
Accompanying this theme, the adult characters seem to be less important and to have less complex (or less thoroughly explored) personal issues. In an adventure story the good and bad guys may be more sharply defined, with fewer shades of grey about them. From my point of view, Campbell, who would probably be the hero were I writing the same story but aimed at older readers, is a lot more of a thoroughly nice bloke than I usually write my heroes.
While I don't see a need to shy away from any aspect of life that could potentially affect my prospective audience, I do find myself writing about those issues slightly differently. What might turn into a secondary relationship in a different version of the story is unlikely to move beyond a deepening friendship with the potential for more in the context of this story. Likewise a lot more will be happening 'off screen' and only be referred to obliquely in the text, although I'm not going to avoid the fact that sometimes bad things happen to good people, or that actions, no matter how well intended, may have unforeseen consequences.
Lastly, I intend this story to have a Happy For Now ending without any major loose ends, although not in a way that excludes further adventures for the characters. There'll be no JK Rowling style epilogue though: I wouldn't want to presume that characters who are perfect for each other at age 15 or 16 will always be perfect for each other (or that they won't): I want to leave some things to readers' imaginations.
How about you? What is it that defines a novel aimed at a particular age group?