You're gonna be bothered by how cool I am.

I felt that Wormtail's hand being chopped off BY HIM was enough to make GoF PG-13 without anything else, although I'm not completely clear why OotP was PG-13. Perhaps Voldemort possessing Harry? The Occlumency lessons? (And the slashy subtext thereof?) So I can see HBP being PG if they do one thing in particular: keep the violence committed by Greyback offscreen and have the innuendo concerning his chief choice of target (young children) vague enough to fly over the heads of the kiddies. Otherwise it's a relatively bloodless affair, IMO, but I'm also guessing that a lot of the sexual innuendo will be cut out or made sufficiently cryptic to younger kids.