From what I’ve heard they removed important character traits from each protagonist. They took away Aang’s playfulness, Katara’s maturity and sense of responsibility, and Sokka’s growth arc.

Or for a quicker summary: they combined the Jet episode with the machinist and the secret tunnel.

It's not so much that they changed things, if anything it's extremely faithful in the overall plot progression, it's mostly just... really shoddy.

Characters are devoid of any personality or life, the dialogue is some of the worst expository garbage I've ever had to endure, and they just keep missing the point of critical moments and character beats.

Like just to give an easy example, upon being told that he's the Avatar, Aang in this version does NOT run away. He just... goes on a little trip on Appa to lighten up, and it just so happens that this is exactly when the fire nation attacks, and he accidentally gets caught in a storm and gets trapped. Running away was key to his character, it's a crucial, character defining moment. It leads into his genuine feelings of guilt for abandoning the world, and his whole arc in the show is about slowly accepting the responsibility that terrified him back then.

And that just keeps happening, super important scenes like that get butchered for no reason, completely erasing the meaning behind them. It feels like they went about the show in a very utilitarian way, believing that as long as they could get the characters from point A to point B, it didn't matter what they changed. The original is so good at that, so good at symbolism, so consistent in its characterization that you're often able to predict how a given character will react because you know them so well.

I think that's what pissed me off the most, and combined with the goddawful dialogue (seriously I can't stress enough how bad the dialogue is), and a lot of gratuitous fanservice (lots of characters and scenes appear much earlier just to show them off lol), and you end up with a show that's extremely hard to sit through if you have any affection for the original.

I watched only the first episode and haven't watched the original in years. But the acting is really really bad. Totally stilted performance. And there are many instances of people exposition dumping their feelings instead of just showing them. And then these parts are underlined with cheesy flashbacks of bad exposition dumps in case someone didn't pay attention.

(TL;DR): Even without the context of the original cartoon, this adaptation has a "college project with a high budget" vibe, like they were in a rush to get it to their professor's desk on time. All of the acting, writing, choreography, and cinematography is mediocre at best and cringe-worthy at worst. The VFX are better this time around at least, but they rarely utilize it where it was needed most and ultimately doesn't outweigh it's shortcomings.

The CGI is way better than the 2010 movie, but all the hurdles that come with live action still plague the Netflix show. They cut a lot of really good scenes because they'd be too hard to animate, (like Aang's escape from Zuko and subsequent first use of his avatar abilities) which creates a lot of pacing issues, and what they didn't cut is plagued by awkward writing and even worse delivery.

Everyone has something missing from their original character that was a key characteristic in the cartoon. Aang is always super serious now, which shows in his fight scenes too and overall removes the whimsical nature of the show... All of the characters in general are now super serious, so the show is pretty devoid of any lightheartedness... Zuko is a lot less measured and he comes across as silly, which is ironic because he's the one character who's supposed to be very serious... Sokka doesn't start out sexist, which removes an entire growth arc from his character and takes away a key dynamic between him and Katara... Iroh doesn't have his iconic wise/disciplined enigma vibe... I could go on.

Little Joel has a video that slaps about exactly that.