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. They even made an effort to cast mostly Europeans as members of the Norther Water Tribe. It might even confirm a persisting fan theory that Kanna was pregnant with Pakku's son when she ran away, making Sokka and Katara his biological grandchildren. Still...you'd think that the two kids would look just a little more Inuit, considering that Hakoda, at least, married a Southern Water Tribe woman.
Fridge Logic: Firebenders now have to rely on a source of fire, like a torch or brazier. These sources are usually exposed, and to put the fire out would cripple the firebenders. So why does nobody ever think to put them out? Despite this glaring weakness, nobody ever puts out their fires when they see the Fire Nation coming, or targets the fires in combat. Master Pakku even tells the Northern Water Tribe to put out their torches so as not to give the Firebenders extra ammo, and no one does! Is there some sort of heroic version of Mook Chivalry going on here?
Fridge Logic: So, Sozin's comet only allows firbenders to create their own fire. Now, Ozai, as the best firebender in the world, can presumably do this already, so Sozin's Comet will give him no benefit. We may have one anti-climactic boss fight on our hands if there ever is a third movie.
Mild Adaptation-Induced Fridge Horror: If Aang is the last of the Airbenders and, as the Avatar, is not allowed to have a family, either he has to follow the rules and let Airbending die with himnote Shyamalan knows that Airbending isn't something one can just learn, right? or he says Screw the Rules, I'm Doing What's Right! towards his people and marries to continue the Airbender line. Or did Shyamalan envision some Jedi-style Ain't No Rule loophole allowing for attachment-free procreation?