Probably more so than anything else is Zuko's scar is going to look more frightening, you will actually see some blood, Stuff like that which isn't much over all but is stuff you won't see in a kids' animated show on tv. So, I don't think a PG-13 is going to make it anymore like the more is going to suck than a PG or G rating. If anything it may give it a little better chance of not sucking because it will be easier to suspend belief in a live action film when you see characters bleeding a bit after a fight.

The parrots thinks the main that is going to matter the most is how much control the creators have and keep on the contents of the film.