Fridge Brilliance: The Northern Water Tribe appear European while most of the Southern Water Tribe look Inuit- except for Sokka, Katara and Kanna (their grandmother). In the show, Kanna ran away from the Northern tribe, so perhaps their different ethnicities are meant to reflect that.
Doesn't work when they're only meant to be about 1/4 Northern Tribe, though - even if you gave them one white grandparent, the other three should mostly counteract that. Which means according to this film: White-Is-Dominant-Over-Other-Races, which equals Unfortunate Implications.
Fridge Logic: The Fire Nation soldiers imprison a bunch of Earthbenders. On a prison of Earth. You have to wonder who is dumber here: the Earthbenders for not escaping or the Fire Nation soldiers for giving their prisoners access to a ton of weapons in the first place.
Worse is the fact that in the prison they had only ONE source of fire to bend off of. Again, not sure who is dumber here, the Fire Benders for making such an exploitable weakness, or the Earth benders who didn't exploit it.