Compare Aang from episode 1 to Aang from episode 61. He goes from a completely immature kid who wants nothing more than to go penguin sledding to a calm, rational spiritual leader who is far more mature than his age. Korra in episode 1 is a brash teenager who hits things and cries when things don't go her way. Korra in episode 12 (the original intended ending) is a brash kid who confronts Amon without any kind of plan, accidentally defeats him when her airbending turns on randomly, and when she loses three out of her four superpowers, she just cries until Aang saves the day. Especially seeing the beginning of season 2, she learned absolutely nothing and didn't develop at all.
Actually look at the development of Sokka, Katara, Zuko, Azula, Toph, Iroh, etc. and you'll see that they all develop throughout the series. The characters of Korra (with the exception of Asami amd Tarrlok), are exactly the same. ****, even Ty Lee, a character that appears in only a handful of episodes, is more developed than a single character in Korra.
I could write 30 pages on this, but I don't think that's what we want to turn the Smash thread into.