I'm watching this video and this guy rips this series a new butthole.
I only made it to book 2 of this video but his point of TLA being both a serialized and episodic show at the same time where the different episodes all contributed to the overarching plot but all explored different characters and themes vs Korra being an entirely serialized show that never really had the chance to deviate from the plot is valid.
TLA also succeeded because it was a kids show with stuff adults could get into to keep a wide audience fully engaged knda like Animaniacs or Pixar movies. Korra wasn't really a kids show at all. It was a show for adults that pretended to be a kids show. Basically the opposite of TLA which is why it completely failed being on a kids network like Nickelodeon.
I feel like Korra targeted the kids who watched TLA and were now adults as well as that show's adult fans and it did this at the expense of trying to be it's own story. It played too hard to nostalgia instead of trying to be it's own show. I would have rather had Aang and co as adults as a show than a show with mostly new characters that's so heavily tied down to the previous series.
This isn't even getting to the fact that TLA was one story over 3 seasons they planned while Korra was supposed to be a single season and got extended and they made it up on the fly. That might have been what hurt it the most.
I wish we got one more show with the gang older but still able to see some of their adventures. Korra still does a great job expanding the mythos and it's absolutely has the better fleshed out villains. No slight to the Aang villains because Zuko, Iroh, and Azula are some of the best written characters ever
the way Kuvira washed her 
tarnished the Avatar name