I was extremely excited in the beginning because the setting felt very well-constructed. I thought that bending contributing to rapid technological progress made a lot of sense to me, and I really like that there wasn't excessive exposition regarding the state of the world.
So much of what they did from that point just felt so corny.
spoiler
- When Amon took her bending only for the Avatar ancestors to pull up and return it like five minutes later
- Raava and Vaatu being just one-dimensionally "good" and "evil"
- The fucking kaiju fight
- Introducing the dictator lady, Kuvira—can't remember how to spell her name—like three episodes before the next arc, telegraphing that she was the next villain focus so hard
- Kuvira then doing the trite "you saved me, but why" and just completely folding on her entire mission
It was just so disappointing for me. I don't know if they got creatively restricted, or if something happened with the original writers or what. I don't regret watching it, but I just wish they had taken or had been able to take more risks.

I can corroborate your last point because I watched both of these series relatively recently, and I also have little to no nostalgia associated with the subject.
I actually used to despise when ATLA came on. At that age, I could never commit to following plot-heavy shows, because I didn't really watch TV a ton (I thought I watched a lot, but I'm learning now as I talk to peers that it was not lol), and the show felt like it was on forever, eating up time on Nick. I finished it up around this time last year and ATLA is now among my favorite shows ever. I continued with TLOK shortly after, and yeah, those were my feelings.
So from my experience, I'm not gonna say it's the whole "growing out of it" thing. TLOK just is a less interesting story, the way I see it.