First I'll address things that I would have hated about this movie were I not a fan of the Avatar: The Last Airbender series and just some random person watching this as a regular movie. Even if this were an original movie, it would be awful.
The acting is terrible. Really terrible. There's many parts where I had to pause the movie and just say to myself, 'Really?'. There's a moment where Katara says, 'Aang, we have to go.' Which, first off was obvious and didn't need to be stated but secondly her delivery was just so stiff and it didn't have any of the urgency that would have made it a semi-tolerable line.
As previously mentioned the writing is very iffy.
There's really no character development through the entire movie. They try to get in Aang overcoming his guilt, but Aang never actually does. He just has the problem, then he doesn't anymore by the time the movie is over. There's no real character progression to that point. You don't see his journey, you don't see him actually go through the process. It's just he has the problem, he's told he has the problem. Then he just kind of decides that it's over and bam it's over.
The special effects are okay, but they're so slow that it's just tedious and there's not much of it so as a mindless action movie it's a not good either.
The worst part is that the bending is so slow. In the series [yeah I know, comparing] it's so quick and it's amazing to watch. The battles flow naturally. In this movie the bending is so slow that it's ridiculous, I could walk up and slap the characters before they even get their bending really going. So even for an action movie it's bad.
I'd go on and on about how many inaccuracies and how much butchering of the series this movie does... but it's all been said before and any fan that's seen this movie already knows what's wrong with it. A lot.
Don't let curiosity get you to watch this, you will just end up angry and annoyed.