girlfriend says: the two white characters there are from the ice nation which is a northern nation so it makes sense that they are white. i would cast them as eastern europeans or something. the other guy is from the fire nation, so it'd make sense if he were darker because it was close to the equator and very hot there.
Actually, the the movie's white characters resemble Inuit tribes in the cartoon. Your girlfriend doesn't seem to understand melatonin or simple genetics. The fire nation is a strict and militaristic society that resembles Japan, the Earth nation resembles China, and the air nation once resembled Tibet. The culture of these nations is a big part of the series, so it is understandable that hardcore fans are upset.
What no one has mentioned so far is that they made one of the main characters - a blind tomboy, a butch, and the greatest Earth-bender of all time - a male in the movie. Also, the Avatar is no longer a vegetarian, and the Earth and Water kingdoms now resemble western Christian nations.