Look, you said you 'genuinely want to hear other's thoughts on this', so I'm gonna take you at your word in good faith and ignore the tags which seem to indicate otherwise. Feel free to reject my opinion out of hand if what I ship scares you so much for some reason, even though this post has nothing to do with shipping--though that would then indicate to me that you aren't actually interested in the opinions of anyone who might disagree.
It isn't meant to be a Team Avatar movie or even an Avatar the Last Airbender movie. it's meant to be a movie about aang, to finally address to grief he's been feeling for the last decade.
So, this is the first major problem--because the only way this works as a premise is if we accept that this is something the original show utterly failed to do, which is something that keeps being brought up and then kicked right back down as a criticism of the original series.
At this point, the show has been over for nearly twenty years. There have been scads of post-canon content of intensely variable quality, including several years worth of comics detailing the years following the war, much of it centered on Aang. If we're meant to believe that the movie, twenty years after the fact, is 'finally [addressing the] grief he's been feeling', then that calls into question why the original show and also none of the comics ever even attempted to do this.
It also deeply calls into question why it's taken thirteen years (if we accept Bryke's assertion that Aang is 25, and somehow still only just 'on the cusp' of adulthood) for Aang to confront this grief and deal with it. Because we are not given any indication by the movie that he's done any of this work prior to it, which is... strange, all things considered. This is a plotline that would have made far more sense for a teenage Aang to be grappling with as he navigates coming to adulthood in a world that has accepted the loss of an entire people and culture because they were left with little other choice in their absence. Especially considering the fact that we're meant to believe Aang mastered the Avatar State (which necessarily involves some level of self-actualization) at the end of the original show. (Also, the fact that he uses it willy nilly like a fancy power-up renders the Avatar State virtually meaningless. It has no weight. We get no indication that he is channeling the lives and powers of a thousand avatars before him. He just goes glowy for a minor boost to his powers, and it kinda sucks some of the magic out of it for me.)
The entire gaang's purpose was to be his friend and support him, which they did.
And here we get to the second major problem.
Yes, the gaang are supporting characters because this is Aang's movie (although in the show titled after him [in fact, an alternative title for AtLA in some other countries was Avatar: the Legend of Aang] these characters got significantly more equitable focus and characterization in even a single twenty minute episode than any of them got in this hour and a half long movie)--but they aren't given any consideration as a supporting cast, they could have been replaced with a gaggle of barely-named NPCs without changing the plot. In fact, the Denied, Taga, and Sonam all get more characterization and plot relevance, which is insane considering how long we were 'waiting' to see the gaang as adults in a story of their own.
And the thing is, we didn't even get scenes showcasing anyone's friendship--not with each other, and not even with Aang! He doesn't have a single meaningful conversation with anyone. Not even Katara, who exists mostly as a wall of 'you're so right baby' for him to bounce his agonizing off of. She doesn't offer any insight of her own, and the one time she tries, he whips around and yells at her that she can't possibly understand what it's like to be a victim of genocide who feels the burden of restoring their culture.
Even though she was, you know, the sole surviving waterbender in the Southern Water Tribe and the burden of recovering and restoring Southern Waterbending fell to and rested on her shoulders. And nevermind that she'd been with him every step of the way from the moment he came out of the ice to the moment when he ended the war and then beyond it. And nevermind that we're supposed to believe they've been together for thirteen years at this point. We don't even know if they're married! We don't know if their relationship status has changed significantly from when we last saw them as children because the movie just doesn't deem it important to communicate to the audience, even though doing so would be hugely informative about both of their characters!
Even beyond that, though, the fact that this is 'Aang's movie' doesn't really work as an excuse when that was a deliberate choice made by the people producing the film. They did not have to wait twenty years and then give us Adult Aang and the Cardboard Cutouts that Sorta Resemble His Friends! And the movie itself doesn't justify its own existence because the plot is a rehash of old plot points from both the original show and LoK, and because of that absolutely nothing it did could accomplish anything substantial. One way to avoid that would've been to tell a story that focuses on the gaang and one of their lesser known adventures, to give us a story that really digs into who these characters are as adults and how they got there from where the series left off with them as children, but the people making this movie weren't interested in that at all.
She split the sea, she had multiple fight scenes, Republic City is credited as her dream. Also the speech she gave to Tagah was so sweet and shows exactly who Aang was to her. Someone that gave her hope.
Except that hope is little more than a meaningless platitude without anything to anchor it to the narrative or give it any weight. And Katara's insane feats of bending fall more than a little flat when, like twenty minutes later, she's in the middle of a city on the harbor crying about not having any more water to bend. It's also more than a little insulting on its face, because what Katara accomplishes means nothing. She brings herself back from the dead, and it doesn't affect her--and more than that, we don't get to see it. She must have woken up to see her best friends, including her own brother, scattered dead around her, and we don't get to see even a second of it. We don't get to see her grief or panic, and we don't get to see her healing them. We don't get to see them finding Aang, their desperation, their fear. We don't get to see how this must have affected Katara in particular so viscerally, holding Aang's lifeless body in her arms for the second time in her life.
We also don't get to see so much as a single lingering effect. No trauma, no discussion, no apology from Aang for trusting a stranger over his closest friends and getting them all killed. Not that any of the gaang expected or wanted an apology or even blamed him for a single thing, because none of them had more than scraps of their personalities grafted onto a generic shell, the design of which had impressively little thought put into them. Toph was virtually indistinguishable from her 12-year-old self except for the fact that she needed a bra now, and Sokka, Katara, and Aang had their unique features ironed out to generic anime faces. The only one who came out of the redesign process in decent shape was Zuko, and that's mostly because they let him have a different hairstyle than what he sported as a teenager. But the boys are all Generically Anime Beefy and the girls are Slightly Muscular But Not Too Muscular and Suki is Sir Not Appearing In This Film (except as a three second cameo at the very end after not even being mentioned when Sokka, Sokka, talked about how the gaang won the war and beat Ozai).
All of this circles back around to Katara being little more than Aang's hype-gf with no character of her own, and it's honestly just not possible to decouple this from criticism of the comics and LoK because they're all fruit of the same poisonous tree. There is zero indication in this movie that any member of the gaang matters or really has a life when Aang doesn't need them to jump up and help him. They don't talk about what they left behind, or what they risk losing by following Aang into the abyss--they don't talk about why they were so quick to come when Aang called, or how long it'd been since they last saw each other (that we were robbed of a gaang reunion scene, presumably because Aang wouldn't have been there, chaps my ass so badly), and again, we get more insight into the Denied and Taga than we get for any of them! It grates! And Katara bears the brunt of it because at least the others didn't already spend the last twenty years having their personalities entirely subsumed by Aang's wants and needs until she had no personality to speak of outside of being his girlfriend/wife/mother of his children.
So that's why Katara fans in particular are critical of this movie, and it truly has nothing to do with ships (except that Bryke of course found time to shoehorn in another pot shot at zutara instead of using that time to do something valuable and meaningful with the gaang and their relationships to each other and to Aang).
