The Legend of Aang is going from a certain point to somewhere with a goal. All the characters gaining and loosing during the show, there are important moments describing the world and building background story, with lots of surprises during this time. Zuko is a brilliant character, so is the story of her sister, Azula; Aang is learning a lot, we meet very unique and special people. Skills are not just appearing; they are results of logic, hard work, teachers, let them be humans, dragons, badgermoles. The final battle is the most intense battle scene I’ve ever seen and even if we have unanswered questions at the end, the series is complete in three seasons with huge lessons and a giant story.
The Legend of Korra tries to focus on the society aspects rather then a story from A to B, due to it’s nature of not having a set table in the beginning. All the questions raised are important questions but all of them are left opened, more importantly: left unopposed.
The Equalists questioned the benefits of the benders – not a single solution for the problem was proposed (that is beyond the part that we never see the problem on screen, ever). When Korra "argues" with Zaheer about chaos natural order, he tells a complete theory which for Korra replies practically nothing apart from a pathetic "but it’s wrong" type response. If a theory is raised, it should be at least opposed, but not left opened, because it will leave the feeling it’s true – and Zaheer’s theories are anything but true.
My other big issue is that skills are just coming out of nowhere. At the end of season one, suddenly, within a few minutes, Korra can repair bending powers out of thin air. At the end of season three, Boulin can lavabend without any former indication or thought about this. Lavabending, for gods sake, the whole lava-bending is an "eeer…no", since even Avatar Roku, a full fledged, all-four-elements master was only able to do it properly in the Avatar state.
Compared to the months Zuko needed to succeed in redirecting the lightning (which he was thought by the one who watched and learned from the ways of waterbending), this is like the D&D leveling up in the middle of a battle. Korra can metalbend for her first try even though she seems to be pretty far from the earthbending skills Toph had when she unlocked them.
And then there’s season 4 with it’s abysmal, broken, boring, predictable, horrible storyline.
A giant mecha. Really? A giant mecha? My only guess is that they want to buy their way into the Gundam fans.
And I genuinely hate the origin story. The thought that people learnt bending by watching and following nature is essential; the part that it was given to them breaks everything.
We saw truly powerful benders fighting side by side and against Aang – have people forgotten bending? Why are there no more master benders in Korra, like Bumi, Jeong Jeong, Pakku, Iroh?
The whole Legend of Korra is sad. It’s depressing, it’s turning a beautiful, strange, unique world inside out; by trying to be "grown up" and "realistic". Some of the prison gear, the way P’li dies, the very part that people actually die by protagonist hands – do we really need this in Avatar world?
Especially after what Aang tried to teach everyone: there is always another way.
There’s one more thing: Korra is not the soul of the Avatar; she can’t be. One of the fundamental ideas of reincarnation is that with each iteration one gets closer to enlightenment. While Aang was told that because he’s the Avatar, he can never reach true enlightenment, that doesn’t mean that the soul of the Avatar is not getting closer and closer to it. During the finale of the Last Airbender, Aang talks to the previous four Avatars from before him. The airbender (the furthest from him back) tells him he should do what the world needs, the waterbender tried to avoid killing, Kyoshi says she doesn’t see a difference between killing someone and letting them fall to their death, but there is a big one: she gave him a choice; and then Roku refuses to kill Shozen. Yet Aang still wants to stay true to himself and wants to find another way – which he does. To me this is a clear path that with each iteration, the soul of the Avatar is indeed getting closer and closer to enlightenment. So… how am I to believe that Korra, is in any way the next iteration of this soul?
It could have been such a different show. Imagine an avatar who has to take the side of a war to keep the balance, for example. Or a world where technology does progresses, and bending becomes a sport or a spiritual journey at secluded areas, leading to people forgetting them, needing the avatar to bring it back. Anything would have been better, than the ones that got written.