I thought there were some bad things with the movie and then coupled with the fact that I would have preferred it M. Knight had kept it true to the series, it produced an unpleasant experience for me.
As you said Airspeed, the script was awful. This is the area where M. Knight must improve upon the most in the second movie. I also found the movie to be almost completely devoid of any character development, which is crucial to all movies, but moreover, is what I think made the series as good as it was. Almost zero attention was given to Oppa and Momo as well.
In terms of the story being changed, I thought that the movie could have been another half hour longer and included so much more content from the series. I also think it was selfish of M. Knight to change the pronunciation of the character’s names just to make the movie seem more like his own creation and not based on something else. I didn’t like that Suki wasn’t introduced in this movie. I didn’t like how the trap was set for Aang to be captured by Zhao (wouldn’t have had to be the archers from the series, but could have been better). I didn’t like that there was no Zhao-Zuko agni kai. I didn’t like the whole dinner scene with Zhao, Zuko and Iroh. I didn’t like how Zhao just out-of-nowhere assumes that Zuko is the Blue Spirit. Without having watched the series, that assumption would have been so unobvious. There are other things I didn’t like as well. The script, by far, was the worst of anything.
Things I did like about the movie: the Blue Spirit, the few parts that did keep true to the series, the acting, the graphics, the setup for the next movie.
Mostly, I think M. Knight just butchered it in making the movie his own. He should have kept it more true to the series and had a real writer write the script.
I do have the power. I have all the power in the world!