To keep this unspoilery, I'll just say that a certain rebellious character and his motley crew, comes off better in animation than live action. Their goofiness and hijinx are suited to the cartoon world but here they just seem bizarre. Plus it's so rushed. Who are all these people anyway?
There's a tension between the elements that really were better in animation vs the live action adaptation. Another way this manifests is in the acting.
The scenes with Ken Leung and Paul Sun-Hyung Lee for example. The former is using a naturalistic style while the latter is still doing the mannered, animation-style acting. They're both very good but seem to be coming from different universes.
However, these quibbles aren't enough to knock the score down more than a couple points from perfection. I'm enjoying the Fire Nation plot. It was a good call to boost that up front. It adds more tension to Zuko's story, wondering how it's all going to shake out for him. And the depiction of the Earth Kingdom is terrific.
After a mere three episodes, this series has done a great job with worldbuilding and establishing the dysfunction that Aang has to resolve.
Air Nomads: extinct. Fire Nation: crazy, and riven with internal conflict. Earth Kingdom: paranoid, with internal conflict of its own. The Water Tribe seems to still have its head on its shoulders but we've yet to see the main part of their society.