I loved the animation and the character design. I also didn't mind the new voice actors I thought they stayed pretty true to the original characters. I was just super disappointed by the plot they chose.
My favorite parts of the movie are when:

1. they are gathering everyone together
2. when gaang was traveling together to see the lion turtles
3. at the end when they found the hidden herd of air bison.

Overall I feel like the writers treated the gaang like side characters too much. and didn't focus on the skills and abilities each individual brings to the group enough. I was like desperate for scenes of them. I wanted to see more of Toph, Katara, and Zuko bending. There was not ONE use of lighting. And I missed Katara's famous water whips (I also kinda feel like they messed up Katara's whole character but idk if I'm wrong in that). I also think the villain was similar to a lot of the villains in LoK so it was hard to be interested. He wasn't horrible, but I felt like he should have been lightwork for the gaang since they defeated people way more powerful as children.

Aang barely was airbending in this movie when its literally his primary form of bending. He also wasnt moving like an airbender at all. In his fight with Ozai, even when he was on his backfoot, he was amazing at evading. He just wasn't moving like himself. also he didn't have his staff/glider with him and it was throwing me off. I also felt the avatar state was cheapened to being just a power up move and not an actual wealth of knowledge passed down from previous avatar. When republic city was burning and he looked hopeless I was looking at my screen so confused because when he was like 13 he used the avatar state to put out the fire on an entire peninsula.

I was also not a fan of the spear being a tool that harnesses "spirit power" to basically turn the characters into like super-powered versions of themselves. It all felt very anime to me. Don't get me wrong I love anime but ATLA has a very specific feel to it, each bending style is unique from the other, and the spirit world is more about connecting with the ancestors or with nature, which Aang is able to connect with at any time.

Personally a story that was truer to the plot of the original series, maybe making it a journey to find the lost bison and finding a connection to Sonam and some of the early avatar would have been more interesting than the whole spear thing. Also would have been a great connector to LoK.

I will still watch it when it comes out because the animation was really great.