Bolin is a Sokka clone who, instead of actually having useful abilities and learning new skills to keep up, relies on his older brother and gets a Special Snowflake bending upgrade for free to sort out his insecurity crisis.
Mako is a (milder) Zuko clone who has no backstory whatsoever except he had to "be bad" to protect Bolin.
Korra is an amalgam of Aang (Abilities), Katara (The problems she faced) and Toph (the other half of the problems she faced and her personality), but has none of the strengths and all of the weaknesses of the said characters. She has more or less no actual backstory that affected her personality.
Asami is a female Mako who is really smart and can kick ass, but has even less of a real motivation to be on the team.
Tenzin is a wise man who still isn't because he is inflexible, the very opposite of what airbenders were supposed to be.
Try to write more about them that ISN'T in their original versions and/or worse than their original versions.
Now here's hardmode: What are their motivations as characters? Tenzin is just a moralfag, Asami hangs around these people because lol friendship. Korra doesn't really have one apart from "I'm the Avatar!", Mako's is "I want to be a better person and I became a cop to have a reason to fight people to blow off steam while being good". That's the best one, by the way. Bolin also doesn't really have a reason to stick around either.