It's not Avatar: the last airbender. It's Legend of Korra, kinda obvious when I put it like that, isn't it? I'll compare the two, since the first had such monstuous sucess.
The legend of Korra is way darker overall than avatar, has better animation, and instead of telling one story over 3 seasons, it tells a story per season. There are four, one for each villain, that represent different points of view. They are:
*Eugenics(in defense of a purer race, like Hitler),
*Theocracy(government by religion)
*Anarchism(no government)
*Dictatorship(strong, authoritarian, repressive government)
This cartoon is aimed at more mature audiences, it doesn't try to be wholesome, opting for the darker truths of reality. A wise choice, since the first is regarded as perhaps a perfect show. It's still very good, but anything is bad compared to something regarded as perfect.
Personally, I didn't like the relationships in Korra. They are all over the place and a small number of characters date within all possibilities. Relationships in A:TLA is not the strongest part, but they didn't feel so rushed.
The music in Korra is very interesting as well, it has a single theme, as the first, but it doesn't change for the most part in rhythm, speed or tone, opting for the same instruments, putting the chinese erhu always front and center, making it feel as if it were a character,
There is conflicting worldbuilding and the main characters represent their shows accordingly: Aang is a fun loving kid, trying to avoid his big responsability, but eventually comes to it. Korra is a physically buff and headstong woman. If you're a fan of Avatar, the last airbender, this is a must watch. If not, is still a very good show, dealing very tactfully with subjects of trust, emotional maturity, politics and trauma.