Actor Allusion: The character played by Sigourney Weaver was called Shipley in earlier drafts. She spends a lot of time in a capsule, is pretty badass and no-nonsense, and knows a lot about extraterrestrial life forms. Are we being told something?
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. It is likely a large part of this was its message, which could be seen as being potentially inspirational to oppressed people within China. Oddly enough, China still allowed the 3D version to be shown even afterwards.
Based on a Dream: The Na'vi were partially inspired by a dream James Cameron's mother had about a tall blue woman.
Extremely Lengthy Creation: James Cameron started to work on this almost right after Titanic 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."
For the Na'vi hair-braid that links to everything: "Bio USB", "Ponytalia"
The planet Pandora orbits is sometimes referred to as "Bluepiter," since it looks like... well, a blue Jupiter (made kind of obvious by its Great Blue Spot).
Fake American: Jake is American, but his actor Sam Worthington is Australian.
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.
What Could Have Been: 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.
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.
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:
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.
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.
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.