Aang, Sokka, and Iroh's names are mispronounced, along with the word "Avatar".
Roku is no longer Aang's spirit advisor. He was written out in favor of his pet dragon.
The Kyoshi Warriors, Bato, King Bumi, the northern water tribe's "sexist" (for lack of a better word) culture, and Jeong Jeong are all cut from the movie, so we lose a sizable chunk of Sokka, Katara, and Aang character development:
Sokka never learns from the Kyoshi Warriors that men aren't the only warriors, and his ego isn't beaten into the ground by his future girlfriend (not that he had that ego to begin with)
As for plot, there was no mention of Pai Sho, so we didn't get Iroh's foreshadowing of The Order of the White Lotus, so if there is a second movie, that's gonna come out of nowhere (if it doesn't get cut >_>). Without introducing Jet and Jeong Jeong, we lose the one episode antagonist from the Earth Kingdom and the good guy from the Fire Nation (one could argue Iroh already fills this role, but he doesn't really do anything except be the laid back old man who gets mad when a spirit gets killed...he doesn't betray the Fire Nation to protect it like in the cartoon either...just gets pissed). So the movie loses the expanded world where it shows both sides in the war have good people, and both sides have corrupt people. Also, without Jet, Ba Sing Se will lose the suprisingly emotional subplot of Jet's redemption...and, if there's a sequel, the finale will suffer for this.
Without Bumi, we miss Aang's advice to always look for a third option, which comes into play everytime he meets the nutball...which serves as some foreshadowing about how he wants to handle fighting the FireLord. Without Jeong Jeong, Aang never accidentally injures Katara, and she never stumbles upon her healing abilities. Of course, we all know how big of a role the healing power plays in Book 2.
What "filler" (a word that doesn't ~really~ apply to Avatar, since the episodes that didn't further the story usually had some new detail come in, or some character development) did make it in was rushed and lost all point. Shyamalan covers "The Warriors of Kyoshi," "Imprisoned," "The Waterbending Scroll," and Aang's portion of "The Storm" inside of five minutes. The Northern Air Temple is only included just to be there...it's really "The Blue Spirit," with nigh a Mechanist in sight. So, we lose out on the "they were planning it all along" when they hint at using his technology to build war airships and it doesn't come into play until halfway through the last season.
As far what did make it in...it lost all meaning and was repeatedly crammed down the audiences' throats. Zhou has to mention "The Great Library" a dozen times...instead the one, forgetable line from the series that the fans would probably forget about until the library comes up in the second season. Ozai is shown suddenly and randomly, without two seasons of buildup before we see his face. Shyamalan thought he'd throw "Chi" in, although it was never mentioned in the show.
The characterization is completely off:
Katara seems more like an underplayed version of the "Ember Island Players" version
Sokka has an actual serious personality...he never cracks a joke and is rarely the victim of one.
Aang...is completely emotionless, instead of the happy adventurer from the show.
Iroh is only remotely like himself when they are in a small village and he mentions to Zuko "there are a lot of pretty girls here".
Zuko's personality, like Katara, is just a toned down version of his in-universe parody.
Zhou...isn't even recognizable. He reminded of the type of wimpy kid that hangs out with the bully in an elementary school show/movie/book....except he somehow commands an entire navy.
Ozai is your typical angry king...nothing menacing or powerful about him. He's just...angry.
Azula...can't really tell how the personality works, since she had one line...but she looks younger than Aang.
Appa didn't do anything notable...he was just there in the background. Along with Momo (who I don't think was even named...)
The humor was almost non-existant (Sokka gets accidentally frozen once, and a guard is chasing down a small child that replaces Haru...the guard claiming "The child was earthbending pebbles at us from behind a tree. ...it hurt
" )
The backstory makes no sense once Shyamalan messes with it...most firebenders can only bend existing fire, except when Sozin's comet gives them the ability to "Ignite their chi". So...how have these guys survived the century of picking a fight with the entire planet? They also stuck the earthbenders in a random quary...on the dirt...by a rock cliff...without a single firebender who can create his own flame. Why did these guys need the gaang to rescue them?
The children actors feel flat...especially Katara, who looks, feels, and sounds like she's reading off a teleprompter. While the older actors aren't ~bad~, the writting for the characters are inconsistent with the cartoon versions. Forget what moviebob said...pass this up.
