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.
The debate over the movie casting was so intense that entire forums were divided over it, with those supporting the casting and those against it being banned for their opinions. Protesters considered those who supported the casting as blind and shrugging off a big issue, while those that eagerly anticipated the movie saw the protesters as not "true" fans.
Firebending only being able to manipulate pre-existing flames ( except in special circumstances). People tend to either be upset that it's not the same as in the show, or feel that it now fits with the other three bending types in that it doesn't create the bendable element.
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. It probably would have been more effective if all 7 benders instead each picked up a moderately sized rock and just threw them.
"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.
Seychelle Gabriel (movie Yue) voicing Asami in The Legend of Korra, a character first intended as a simple Romantic False Lead but was changed to become much more prominent when she became popular with the fans.
Internet Backdraft: Online criticism of the film reached apoplectic levels. It's a good thing that capital letters and boldface are unlimited resources.
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◊.)
There is one scene at the beginning where Sokka is splashed with water and then is immediately dry. Also, it's an arctic area and you can't see anyone's breath.
Also, the waterbending is sometimes not even animated.
The firebending is pretty much nothing but poorly composed live-action fire elements with some CG that even the film version of Spawn would reject.
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 (despite most of the names being made up) than the Americanized pronunciations from the cartoon. Fans were not pleased.
The Scrappy: 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.
One The Agony Boothrecapper, objects to what they consider a sexist tone in the film, pointing out that a number of important moments for female characters (Katara's speech to the imprisoned earthbenders, Yue realizing she can sacrifice herself to restore waterbending) are given to male characters instead.
Aasif Mandvi as Zhao. Mandvi is a comedic actor best known for his work on The Daily Show.
Many fans of the original cartoon preferred Asian actors to play Katara and Sokka because although the characters have ambiguous racial features, their culture is based on Inuits. Caucasian actors Nicola Peltz and Jackson Rathbone were cast instead, but all the Waterbender extras were Asian or Inuit actors, so the main characters don't seem to belong to their own tribe. The claims that Peltz and Rathbone were simply the best actors for their roles are discarded out of hand, with detractors claiming their acting is terrible.