Casting: Decent, I suppose. Child actors are always a struggle, but these guys are doing pretty well!
Visuals: Pretty good, it's cool to see some iconic places in CGI. But, it's all very obviously green screen, especially the overly clean clothing in the Southern Water Tribe. They feel so separate from the world. It's the same with a lot of the props, they're all so perfect and clean, like the production design team was scared of dirtying up their hard work. I liked the Ottoman vibe for Omashu.
Music: good, I like that they added to the original soundtrack.
Writing (This is where I rant.TL:DR, It's like they never took a class on writing): Quite poor. Firstly, there's an insane amount of exposition, and "As you know," It's all very 'tell' and not 'show.' Characters explain their goals, fears and traumas so blandly (and so often?? It's like a broken record sometimes) and in your face that there's no room for subtext or guesswork, the show feels like it's trying to hold your hand the whole way through. "He's the Avatar, by the way. He wants to save the world." That works if you're aiming it at young kids, but... the show shows the genocide of the air nomads, and has people burning alive on screen. Also, if you're aiming it at kids... kids can still pick up on subtext.
I'm not mad that changes have been made to the original, I know it's not a 1:1 remake. But most of the changes they made are questionable at best and terrible at worst.
Aang 'going for a fly to clear his head' takes away a core part of his character.
Bumi getting angry at him for leaving them all to fight the war for 100 years would have been more impactful if Aang, like in the original, had actually ran away from his responsibilities. Instead, you hear Bumi say that, and you think "Wait, but it wasn't really Aang's fault."
Aang also dives straight into his Avatar responsibility. I understand they're trying to compress a lot, but a part of Aangs character is that he is a reluctant hero at first. In this remake, he may SAY he doesn't want the responsibility, but his actions don't match that. In the OG series, he's off goofing around for ages (while of course, having teachable moments)
Katara feels stripped of all character. She has her trauma, of course, but Katara is kind of naggy and maternal.
Sokka not being sexist pulled all meaning from his interactions with Suki, and took away a very valuable teaching moment that the original had. This show is supposed to be a more 'mature' version? Yet the original can teach that societies can have problematic mentalities towards certain groups, (In the waters tribes case, women) but that you can rise above those and change. And this version is scared of showing any sexism? Like the real world doesn't have that? If 11-year-olds could handle it back in 2005, I'm sure a mature audience should be able to handle it now. (Then again...)
It's like they're terrified of making any of the characters controversial. Someone said in another review that the characters have more backbone... I personally feel like they've been stripped down to the blandest archetypes possible.
Some of the other changes are fine. I quite like Azula's entrance being bought forward. And I find the manipulative 'pitting his kids against each other' aspect to Ozai fairly interesting.
Yeah, anyway. The writing is the weakest part for me. They've tried to cast the net for an audience as wide as possible, and in doing so, they've left the story hollow.