They've been really enjoying the show, especially because of its attention to detail and world-building. Also, the characters are very well thought out and have personal growth.
Stephen's one complaint about the show is that ATLA is a kid's show. Since the context of the show is pretty dark, he wishes that it could have explored some of the mature themes being hinted at. He also doesn't appreciate the moral lessons very much as an adult, but he does appreciate having this kind of a show for kids.
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That being said, towards the end of Book 1, they felt a tonal shift away from imparting moral lessons to exploring more of the mature struggles that the characters are facing from being in the midst of war.
In other news, they've been having trouble remembering the characters' names, but they do remember Momo who apparently is "just Keppy", according to Mal.