For the Southern Water Tribe and air nomads, a lot. Only one southern water tribe remained after the war; and the air nomads were completely wiped out, other than Aang. In a way, the Fire Nation committed genocide against the Southern Water Tribe, wiping out all but one southern water bender.

Meanwhile, very little affected the Northern Water tribe for the majority of the war, the first invasion having been a miserable failure and the second also seeing defeat at the hands of Avatar Aang. The only true change was the death of the king's daughter, who turned into the moon, leaving no rightful heir to the throne.

Lastly, the Earth kingdom. Their main trade partner, the Fire Nation, had destroyed contact with their other main trade partner, the Southern Water Tribe, and embargoed them. Economically, the entire Earth Kingdom struggled (in a way the Fire Nation did too, at first, but the industrial revolution they experienced fixed most of their problems). Over the next century, the Earth Kingdom would see seizure of their northwestern lands and southern lands the most, all slowly creeping towards the city of Ba Sing Se.