Accidentally Correct Writing: In the movie, Pandora is a moon orbiting the gas giant Polyphemus which in turn orbits Alpha Centauri A, the main star of the Alpha Centauri system (which is the nearest star system to the Sun). Flash forward 12 years later and NASA seems to have found a gas giant orbiting Alpha Centauri A in the habitable zone. In short, Polyphemus was defictionalized, and it's possible that there might just be a Pandora orbiting it just as in the movie. Whether we're going to meet actual Na'vi there is, of course, yet to be seen.
Banned in China: It was released in China, but its 2D version was pulled from cinemas very quickly afterwards despite the film being the most popular shown in China ever at the time. It was long rumored that it was because of its message, which could be seen as being potentially inspirational to oppressed people within China, but a more likely reason was that it was eating into the profit margin of a state-sanctioned biopic of Confucius starring Chow Yun-fat that was running concurrently. The 3D version was released there without incident.
Based on a Dream: The Na'vi were partially inspired by a dream James Cameron's mother had about a tall blue woman.
James Cameron started to work on this almost right after Titanic (1997) was finished in 1997. Unfortunately, because he kept waiting for the technology to catch up to his vision, people started to place it on lists of "movies that will never be made."
Follow the Leader: Not necessarily in terms of style or plot, but a number of movies that had been filmed in 2D were hastily retrofitted with an additional dimension in the wake of the Avatar phenomenon (including Cameron's own Titanic).
Hey, It's That Sound!: The odd, barking "cough" sound made by the dire-horses is identical to the calling sound the raptors make in the first Jurassic Park film (watch in the kitchen, when the raptor calls for its buddy to look for Tim & Lex). Of course, they should sound the same: both are the cough of a male walrus or elephant seal. Some of the thanator's roars seem to be the same as those of the T. rex, as well. Both films used the same SFX shop.
Killer App: For the Blu-ray format. It sold 1.5 million copies on release day, 6.2 million after three weeks. For quite a while, you couldn't walk into a Best Buy without seeing it all over the TV wall.
Updated Re-release: The movie was sent back to the theaters after its original premiere with new scenes.
Vanilla Edition: The Earth Day release of the movie has absolutely nothing outside of the movie, main menu, and an options menu. Even the 2-disc BluRay set. Then Fox releases a "special edition" set in late 2010...
James Cameron originally planned on filming Avatar immediately after finishing Titanic. He soon found that the special effects technology of the time wouldn't be sufficient for portraying Pandora.
Project 880, James Cameron's original script (here; summary here). Some items of note:
Earth and its environmental problems are explored. (Sully has never even seen a forest, so Grace has to practically hold his hand when they go into the woods.)
We see Josh (Jake) Sully's Avatar being born — Sully actually "births" himself. Also, his reaction to walking again is quite different: it takes him a while to gain any sort of strength, and then he cries.
Instead of being mindless inert bodies, the Avatars are depicted with a rudimentary consciousness similar to newborn babies, able to cry or reach out to their Comtrollers for comfort.
It's revealed the Avatar program originally existed to train Na'vi to be an indigenous workforce for the Corporation, since it's so expensive to send human workers. Obviously, they didn't like that.
There is an Avatar controller who is burnt out because his Avatar died with him in it. He committed Avatar suicide because he had fallen in love with a Na'vi girl who had been killed by the military.
The Avatars have a Na'vi guide named N'Deh who is sleeping with Grace.
Grace survives the soul transfer.
Many of the creatures are significantly different in the script. The banshees are more like manta rays in their description, while the Thanator is instead called a Manticore and possessed a venomous stinger tail.
Some creature concepts were entirely cut from the film, such as the Slinth, a panther-like creature with a spearing head, or the Snake Tree, a hydra-like carnivorous plant with six toothy heads.
The Banshee's do not bond for life in this script: they fly away once they disengage from the rider. Also, the entire concept of "Toruk Makto" is non-existent: the need to tame one to regain the Navi's trust was never in the original script, and both Jake/Josh and Tsu'tey get to ride on a mated pair of Leonopteryxes both of which are unfortunately killed in the final battle.
Michael Biehn, Cameron's old go-to-guy from the '80s (Kyle Reese in The Terminator, Dwayne Hicks in Aliens, etc.) was the original choice to play Colonel Quaritch but Cameron decided not to use him because he thought would have made the film feel too much like Aliens (even though that film, while a classic, hadn't really been relevant for like 20 years by the time Avatar got made).
Brie Larson auditioned for a part (though she doesn't remember the audition).
An entire musical culture was created for the Na'avi. Complex orchestrations were created in this alien style (that took into account the fact that the Na'avi have four fingers) and songs and chants were developed in the Na'avi language with lyrics that had cultural significance unique to the tribe primarily seen in the movie. Unfortunately, James Cameron struggled to comprehend the final product because it didn't sound "right" to ears familiar with European musical styles. So, most of the work was abandoned in favor of "traditional" orchestrations and almost all of the meaningful Na'avi lyrics were swapped for nonsense syllables that fit the music while sounding sufficiently alien.
Since the movie employs basic tropes in a story plotted in a simple way, it is possible to imagine it to be a lot like other stories. It isn't really any more or less "derivative" than anything else. This is the first movie on TV Tropes that has sparked people to list so many different stories the movie reminds them of. A look at the list may reveal something about the themes and elements that stick out in people's minds:
Foremost is the film to which Avatar is the most direct Spiritual Successor, Return of the Jedi. The main action takes place on a forest moon far from the core interstellar human civilization; many names are similar (Endor/Pandora, Ewok/Eywa, Solo/Sully); there is heavy emphasis on practical mysticism (The Force/Eywa's "network"); and the plot involves a mighty, technologically advanced force occupying a world for sinister ends, and being overcome by "primitive" natives in alliance with rebel forces. The final battle revolves around a large superweapon that must be destroyed from the inside. Had Lucas gone with his original plan of making Endor's inhabitants towering Wookiees, the similarities would have been even stronger!
Transform the protagonist to resemble the invaded, non-human natives, add a deep mystic connection to their environment on a spiritual AND physical level, villains dead set on exploiting natural resources for financial gain, and an environmentalist message and we get FernGully: The Last Rainforest.
Albion has a very similar plot, a planet with large 'network of energy' that connects all living beings, and even the natives of the planet look almost the same (only smaller and less blue), they even have a specialized organ that allows them to telepathically communicate with each other (a crystalline formation on their forehead). The game was released around the time Cameron first conceived the plot for this movie.
A Death World is the only place in the universe where one can find an extremely costly substance that's important for interstellar travel. An off-worlder shows up and finds a place among the natives, eventually leading them to victory over his people. Bonus points for him fitting their legend, riding a giant powerful animal. Double bonus for having an ecologist working for the bad guys but in essence one of the good guys, who also scoffs at the bad guys for their industrialist interest and missing the beauty of the planet itself. Anyone still question Dune was an influence here?
A disabled man goes to a distant land and meets some miners who are trying to mine precious minerals from underneath an ancient forest being guarded by intelligent native creatures. He also meets many of the forest-dwellers and becomes friends with them, especially a certain young woman. Then the miners and forest-dwellers get into an all-out war where many are killed while he tries to stop the war. Yep, it's Princess Mononoke.
The little-known 2001 Italian animated film "Aida of the Trees/Aida degli alberi", very vaguely based upon Giuseppe Verdi's opera Aida, features no humans, but two warring populations of humanoids with feline features. Arborea is peaceful and resides in huts on the branches of a giant tree, Petra is militarized, technological and wants to usurp Arborea's resources. As the article linked in the Italian wiki page for the film shows, there are a few visual similarities as well.
A disabled employee of a powerful interstellar company goes to a hostile world to find its secrets and discovers that the natives have very close links with the planet to the point that said links are almost psychic, that the planet itself is sentient, and that the planet opposes the planned mining exploits to the point that it starts fighting back. She also becomes good friends with the natives, falls for a particular man who has a rather important role there, and her disability is cured by the end of the book- it's Anne McCaffrey and Elizabeth Ann Scarborough's book Powers That Be.
Dueling Movies: By sheer accident or twist of fate, several movies have come out since production began on Avatar that are somewhat similar:
Planet 51 also dealt with the "human as alien invader" story (as a comedy, of course).
Delgo, another movie about two different sapient races coming to blows over... something, to the point where the creators of the former attempted to sue Cameron over the production of Avatar.
What Could Have Been: Avatar was to have been designed by Steve Ritchie; he pursued the license for that game. After his first layoff from Stern Pinball, Steve posted his design blueprint on his website.