Fire Nation:
Lighter skin
Black or Brown hair
Amber, Gold, Brown, or Grey eyes
Men sport mustaches, sideburns, and beards--wear hair back
Women sport longer fingernails if nobility, wear hair back.
Both genders wear topknots, decorated with ranking fire emblem: the Firelord will always have five prongs. The members of his council have four. The members of nobility own three, middle class two, and normal citizens a single prong.
Clothing consists of reds, blacks, grays, pinks, whites, and darker browns.
Heavy Chinese Qin, Tang, and Han dynasty period influence.
Names typically have "Z" pronunciation, with heavy Chinese influence.
Earth Kingdom:
Darker skin: Nobles will have lighter tones.
Dark brown or black hair
Green, Brown, or Grey eyes
Men sport mustaches, long beards.
Women wear hair up elaborately, or down.
Clothing consists of greens, yellows, beiges, whites, and browns.
Air Nomads
Lighter skin tones
Dark brown, or black hair.
Blue, grey, brown, or green eyes.
Older men sport long mustaches and beards, heads are shaven fully.
Women shave only top part of head to show arrow.
Clothing consists of oranges, yellows, and browns.
Heavy influence from Shao Lin monks, and Tibetan monks.
Water Tribe:
Light brown skin tones
Light and Dark brown hair.
Blue, grey, or brown eyes.
Men sport "warrior's wolf tails" when they are younger, grown out longer after trials.
Women sport braids/plaits in various styles, as well as "hair loopies"
Clothing consists of light and dark blues, browns, whites, and purples.
Northern Water Tribe members wear darker blues than Southern.
Royalty typically wears more purple, and no brown.
Names typically have "K" in it, and "AH" in the pronunciation.
Republic City:
Steampunk, steampunk, steampunk! The Avatar world is on the brink of its Industrial Revolution!
Everybody from every nation lives here, so the possibilities are quite large here for just about anything.
In a gang? Think "Chinese Mafia."
We're not going to hound you if you don't have these details in here, but PLEASE be aware that each nation has its own demographics and styles as far as people go. It's going to be a rare time where there's a Fire Nation kid with blue eyes, or a Water Tribe girl with red hair. It doesn't mean you can have it, but please be aware of it.