These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: The Last Airbender
Adaptation Expansion: The novelization had a lot less cut from it and was said to be far superior in comparison. Though it's not like this movie set the bar that high.
Awesome Music: James Newton Howard's score is quite possibly the best thing in the entire film.
Zuko. He goes from the angsty but determined powerhouse in Season 1 and turns into a whiny, Pirates Who Don't Do Anything type. His Crowning Moment of Awesome from Season 1, defeating rival Admiral Zhao and proving his worth as a Firebender, is instead given to a handful of random Waterbenders. And unlike Zuko, they KILL Zhao.
The Fire Nation in general. Not only are its soldiers somewhat wimpy ("It really hurt!"), Zhao finds more time relying on the Fire Lord (and the scroll from the great library) rather than be ambitious and power-hungry, FireLordOzai is not as menacing as his animated counterpart.
Firebenders can only manipulate pre-existing flames. Sozin's Comet, which previously allowed firebenders to wipe out entire civilizations in a day, now apparently only makes all firebenders roughly equivalent to a normal, non cometed firebender in the series.
While not a Badass until the end of Season 1, Katara underwent this via Chickification, more closely resembling the FlanderizedLove Freak that the show itself parodied in "The Ember Island Players" than her character in story.
Sokka in particular is much closer to his initial idea of being a wannabe macho-man twat as opposed to a clever funny guy, and doesn't crack any jokes throughout the film, or even much of a smile. Sokka's fans were equally unamused.
Complete Monster: Fire Lord Ozai, like always. Like in the cartoon, he burned and banished his son for speaking out of turn. If that weren't enough, in the novelization, HE was the one who ordered Zhao to blow up Zuko's ship and make it look like an accident.
Fanon Discontinuity: Ask a fan of the series if there is a Live Acton film of the first book. They will deny it.
Well they will admit there was a film focusing on the Ah-vatar with Oong, but no film about the Avatar with Aang.
Fan Dumb: To quote many a message board "Where's TOPH?! I'm not watching this because she isn't in it!" (For those unfamiliar, Toph joined the main cast in Season 2, after the material this film is adapting.)
From the same scene, Katara pushing one soldier to the ground and then just standing there doing nothing else is called the Ka-Tackle.
Fight Scene Failure: All of them, but the most infamous by far is the Earthbending scene (which is the only one, aside from the intro, to have Earthbending).
When the Earthbender prisoners are staging a revolt, we see six Earthbenders moving in perfect precision and choreography, stomping the earth and releasing a mighty battle cry... and a tiny rock barely bigger than a football floats by very, very slowly. A seventh Earthbender then launches it at a guard. A fan theory suggested that the dancing earthbenders were actually responsible for making the earth walls previously used to block the fire blasts, but the scene alludes to the cartoon, in which Haru's father was the one who made the earth wall.
Worse, the fight scenes are all shot with a single camera, with no jump cuts. This leads to the problem of showing the audience a large number of mooks standing in the background, not doing anything while their friends are getting knocked senseless.
Hilarious in Hindsight: The Ember Island Players episode which came out at the time when this movie was in production. Needless to say, a lot of jokes were to be had using footage from that episode to showcase how bad this film was.
Memetic Mutation: Before the movie had even come out, early test screening reviews mentioned a scene that has turned the phrase "punching the fish" into a meme◊note He actually stabs it with a knife.. Other gems include the pebble dance (the most notorious example of the Flynning mentioned abovenote Six earthbenders. Four seconds of kata. One rock, slightly larger than a human head.), Kat-tackle (Katara pushing over a random guard and then just standing there), Zhao constantly mentioning the secret library, Ozai's glowing ass (from a screenshot in which the source of light appears to be Ozai's rear end), Yue's line about believing in beliefs, and her penis hair (how it looks like when viewed from the back◊.)
They Changed It, Now It Sucks: Long before the movie was anything more than a casting list and a few trailers, fans were complaining about changes to the source material. One of the biggest examples from the movie once it finally came out was the pronunciation changes, which Shyamalan deliberately included to make the character names sound closer to their "correct" Asian pronunciations than the Americanized pronunciations from the cartoon. Fans were not pleased.
And they weren't pleased because A. "Ah-vatar" doesn't exist because Sanskrit (the language that the word Avatar comes from) doesn't have a stressed "Ah" sound in it and B. the changes in pronunciation were arbitrary and downright stupid (where in the bloody hell do you get "Ki" from the word "Kai"?)
To add a little extra Wallbanger, of the three leads only Aang has an "Asian" name being a Fantasy Counterpart Culture Tibetian. Both Sokka and Katara are supposed to be Inuit.
The Scrappy: Film!Katara has far less fans than her cartoon counterpart, mainly for hardly being like Cartoon!Katara and her actress having a severe case of Dull Surprise. It also doesn't help that many of her strong character development moments were either given to Aang or just removed entirely.