TLA and LoK are two very different series that have a very different vibe. TLA was more humorous and in fact more 'childish'. The main protagonists were also children/early teens. LoK on the other hand is more serious. It has less humor, the main characters are late teens/early adults and the underlying themes are often more complex.
I always had mixed feelings about the modern setting, which is based on the years 1920-1930. Still it has some kind of charm. Because it's so modern, it's much more easier to be familiar with it on a personal level and it makes you start thinking about how the world could have looked if some things were different. They integrated something like bending, a concept that does not exist in the real world, in a society that's a copy of our own. I find it fascinating to see how that works out and especially the question whether benders are still needed in this current society, which is the central theme of the first season, is something that could be taken out of today's newspaper. Not the part of benders of course, but the questioning of ideas, concepts and traditions that have been around for a long time and whether they still should be used in the current era, is something that's happening all over the world. You can mirror our own world to that of LoK, something that is much more difficult with TLA. This is really heavy stuff you know.
But in the end whether you now like this setting or not, is a personal choice. There isn't a right answer on it and it can also not be ignored that LoK needs TLA as without the latter, the first loses part of its character and depth. In the least however you need to give credits to the makers that they didn't try to just make a copy of TLA. They could have easily used the exact same formula of TLA, but they decided to make something different.
I loved seasons 1 and 3, they especially did a terrific job at creating the two main villains: Amon and Zaheer. Season 2 wasn't my favourite, despite it having some über-excellent episodes on the origin of the very 1st Avatar. My main problem with this season was though that it was too bombastic and pulled out of proportion. Realistically speaking all the problems Korra and Aang together had to deal with are peanuts compared to what Korra had to do in the 2nd season. It simply doesn't fit, it feels like an anomaly, like the 2nd season isn't canon.
Either way in the end LoK isn't really that long, especially when you compare it to the amount of episodes OP has.
