The Last Airbender/WMG

The Firebenders were Able to Conquer the Other Three Nations by Bending Magma.
Without the ability to create their own fire, the Firebenders (or, at least, the most powerful ones) had to learn to summon the magma from the very core of the planet. With this power, they were able to take over the world by flooding town after town with molten lava (as well as the fires that they create). Additionally, they're also capable of erupting and then controlling volcanoes- specifically, supervolcanoes, which literally have the potential to wipe out the entire human species. Given all these powers, it's easy to understand how Firebenders were able to get the other nations to surrender.
 * Granted, magis arguably more earth than it is fire, but then again, where there is lava, there is certainly fire, and it's plausible to believe that the Firebenders are more powerful than the Earthbenders, so their control over the substance would be more powerful as well.
 * Considering earthbenders are apparently very, very stupid, based on the prison camp, it doesn't seem like the Fire Nation would need anything more than pointy sticks to conquer them.

The word "Avatar" is blessed as a title.
A cartoon and a film with "Avatar" in/as the title became instant sensations. The Last Airbender failed because they dropped the magic word from the title.

The 'friends' that were to be sacrificed during the battle were Mai and Ty Lee
During the scene when the boy and Zuko are relaying the story of the Fire Nation prince, Zuko mentions that he was trying to defend his friends from being sacrificed in a battle. What better bait than a pair of children of influential Fire Nation aristocrats?

Alternatively, Avatars are not allowed to have a family, but Roku broke the rules.
With disastrous results, as his desire to settle down with a family prevented him from taking the necessary steps to preserve peace, which would have required a much greater time commitment.

M. Night's real twist is that Zuko joins Aang rather than Azula at the end of the book 2 movie
It would help cut out a lot of what he would need to do to adapt book 3 into a movie by not having to show Zuko going back to the Fire Nation and could jump into Aang's firebending training right away.

The cameraman was Koh.
That's why the acting is so dead; they literally couldn't show any expression to keep from having their faces stolen.

Shyamalan was attempting to make the anti-Ember Island Players
He just went too far in the other direction. Thus, movie-Sokka lost the comedic side of his character, movie-Aang lost his bounciness, movie-Katara never got to make her stirring speech about hope to the imprisoned earthbenders, and Movie!Iroh is skinny.

Movie!Iroh got his dreadlocks during his spiritual quest.
This assumes that the movieverse Fire Nation has Indian influences beyond Dev Patel and a few references to Agni. In some strains of Hinduism, dreadlocks are associated with asceticism and spiritual power, and the Vedic "Hymn of the Longhaired Sage" mentions mastery of fire.

The Film is a case of Unreliable Narrator.
Obviously it is in no way connected to the cartoon but instead is a story about a plucky nation deciding they no longer wish to be under the oppressive thumb of the Spiritworld and by definition the Avatar, only the story is told from the point of view of the oppressors. This way it is logical why the Earthbenders are kept in an open place with no attempt to cover the ground with something they couldn't use, the Firebenders kill people (it's a revolution after all), but see no point in killing needlessly. Also explains why we never really see the much talked about Machines and why the most sympathetic people in the film are the guys playing the Firebenders, it was genius casting by M.Night. The entire film is in fact a propaganda story told by the supporters of the Avatar, Katara and Co might never have actually existed and are simply badly thought through characters in the story.

Shaymalan thought the real hero was Iroh.
He was certainly more heroic than any of the actual protagonists. His giving Yue the idea for her Heroic Sacrifice rather than it being her own choice all along was just the icing on the cake. Movie!Iroh was more Qui-Gon Jinn than the tea-crazy, wise-but-world-weary Defector From Decadence that ATLA!Iroh was.

Shaymalan intentionally made the world's most elaborate Fix Fic.
The sheer number of things that were changed and cropped out make little sense, for such a supposed "fan" of the series, other than he wanted to do the Avatar story his way - the "right" way.