Another needless change: Firebenders now need a nearby source of fire to bend. They can't generate fire by themselves (unless under special conditions). Not really a huge change, but I'm scratching my head at why they made the change at all. The show managed to work it in and not have firebenders be overpowered.
Actually this makes sense to me. I mean, the other three kinds of benders don't generate their elements out of their own bodies (except for Katara bending her own sweat, which isn't quite the same thing); as a rule, they manipulate the pre-existing substances in their environment. In the show, the Firebenders are the exception to that rule. That's somewhat justified by the identification of chi
with energy and energy with fire, but the movie's approach here just seems more self-consistent.