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.