The movie itself isn't as bad as the hype makes it out to be. What makes it that bad is the fact that its also based off of a series that it didn't stick to.
Average Movie
I'm better at critiquing games than movies, but here's my jist. The movie is slightly above average if you take out the stuff based off of the series. The acting isn't bad to me. However, the plot was very rushed. One minute wannabe team avatar is at the tip of the Earth Kingdom colonies, the next they're at the northern water tribe. Narrating had to be used to tell what were very important parts. This took away from the feel that you were traveling with Aang. It was very hard to relate with any character as you don't spend enough time learning why they feel what they feel. The CG was excellent, though it seemed alittle out-of-place-ish. Appa just didn't seem like he belonged with the group.
Avatar Movie
This is where the movie gets all the disgust it deserves. It simply did not stick to the Avatar feel we were supposed to get. First off, obvious point, the pronunciations were changed. I saw one comment that said the movie did this to feel more real. I believe that there's a correct way to try any idea, and this was not it. Even if that is how it would be properly pronounced, that isn't the way people know to pronounce it. Society has an idea of what every character's name should sound like, so when you replace that, everytime they hear the name they will have trouble actually identifying the character. While watching the movie, whenever someone says "Sohkah" you think "IT'S PRONOUNCED SAHKAW" and take yourself out of the movie, disrupting the flow and bringing to the realization that what you are watching is not real.
Plotwise, this suffers. I realize that the movie was on a time limit, and even though I can't think of a way to put all 20 episodes of book water into the movie, the way it was done sucked. The fact is, Avatar the series is very patient with its world. Throughout the series, you had time to learn exactly what kind of world you were living in. You learned that the Earthbenders were proud people, that water benders were very emotional, and that firebenders were very stern. You learned what places mattered and what places were just rinky dink towns. Overtime you saw a platypus bear and learned that it was not a very rare animal, but rather very common. The movie does not give you this time to learn the world nor the characters and thus you cannot take yourself out of the real world and put yourself into that of the Last Airbender's.
The Personalities. I feel this is where the movie suffered the most. What the Last Airbender did was completely change the source material. You can argue the name change and I'll listen, but I'm sold that changing the Personalities was a horrible decision. Aang is no longer the happy go lucky child we know, but a solom kid with a grim attitude towards too many things. In the series Aang could also be grim, but he kept it inside as his Personality was to remain positive even when the chips were down. Sokka is way too quiet. When he speaks, his Personality fits, but the fact that he doesn't speak that much goes against everything Sokka is. And Zuko OH EM GOODNESS! Zuko is angry all the time, that's who Zuko is. He deserves to be angry because no matter how hard he tries at life, failure continues to rear its head. No body helps him even when he does good except his uncle. The boy on the screen was way too calm to be Zuko. Without that temper, he's just another guy who had something bad happen to him. Changing the personalities as drastically as they did completely took away from the story. Observing the show's Personalities lead to alot of depth discovery to the character which was thrown away in the movie. It doesn't create new depth either, it creates an unwanted and unnapreciated clicheness of stereotypical characters. I think this is why alot say the acting was bad. Because the characters are very cliche as well as they go against their Personalities in the series.
The Avatar concept itself. Too many things. Why change the style from martial arts to dancing? I really don't understand that. The fact that bending was martial arts made the war more real for people. This is a story about fighting, naturally people should be fighting. Because of the dancing, the bending seems fake to people who've seen the show. Not to mention, the dancing didn't really correlate well as it seemed to be ruleless (though I choose to believe their was some kind of rules). I didn't understand how Aang's dance could create a small tornado. It didn't seem like he was bending the air, but rather the air was just moving in front of him. The show had rules. If you didn't move correctly, as well as have the right attitude, what you were doing would not work
The concept that fire can't be created except by the strongest firebenders takes so much away from the series. That's one thing that made firebending unique. No one could create other elements via bending, yet there was plenty of it to go around. It doesn't seem to make sense that firebenders could fight a war this long if they couldn't create fire as most of their machinery in the series, which gave them an advantage in the war, came from their ability to create fire.
This also includes benders not bending. One thing about bending that the cartoon series showed was that it wasn't just a fighting tool, but a way of life. You constantly saw people who could bend do it, even when they weren't supposed to or allowed to, and that's because it was a part of them. The movie had scenes where benders just simply weren't doing it. Of course the Earth Kingdom prison comes to mind, but mainly I'm referring to the fight between Aang and Zuko. Aang ALWAYS airbends, and Zuko only doesn't bend when he's in disguise. So why was there a fight scene where neither of them bended at each other?
I was also upset, though not as much, to see the change in races. In the series, water tribe is Eskimoes. Air Nomads are Indians. Earth Kindom is Chinese. And the Fire Nation is Japanese. The change in the races takes away from the hidden meanings behind each nation.
And there are more, but those are the main ones. If I say everything, I'd be here all day.
All in all, I must conclude that I while I did enjoy the movie as a movie, it wasn't as near as good as it could've or should've been. There was too much taken away from the source material to the point where I didn't feel like I was watching a movie about the avatar, but rather someone's bad alternate dimension version. I realize that from source material to screening, things will change, but this movie made changes in the wrong places. It felt like someone watched a fastforwarded version of book water and then tried to make a movie off of the parts they were able to catch.