Here's a brief outline of the story: 2154. Earth is desperately looking for energy resources. A distant space body called Pandora has this rich mineral, funnily titled, unobtanium. Corporations and military send a force to study the natives of Pandora, negotiate a displacement to mine the mineral under their, wait for this, sacred tree. If negotiation doesn't work military might will have to be sought. (Why only the U.S military if the whole of Earth needs energy resources? China already bats towards imperialism. I would have appreciated Cameron if there had been a racial/geographical medley instead of just American soldiers. We see an Asian scientist, but he finally turns out to be a good guy).
I'm not a fan of good vs evil stories painted in broad strokes. You can make a movie appealing to anti-war and go-green activists, but this one is thematically immature to have a meaningful conversation about them when stepping out of the theater. (Ironically though, it has borrowed concepts from The Matrix, Dances with the Wolves and The Last Samurai, all of which do a decent job of getting the audience to delve into their worlds). Spielberg once said that visual effects should help the story, it cannot be the story. He also said that many give credit to Cameron for the technocrat he is but not the story-teller. I agree with the 1st sentence, not the 2nd one.
But go see it in 3-D for the visual orgasms it has to offer. This I like very much about Cameron - being able to realize the surreal imagery in his mind onto the screen. The world of Pandora is spectacularly vibrant, colorful and interesting. The middle segment is spacious and sets up the bond between the hero (a bio-engineered part human part native) and heroine. Cameron's not Michael Bay to throw up an action sequence once every 20 minutes. When there are no fights, there are adventures. We see new things along with the hero. This is a sample entrée in the banquet for my fantasy taste buds: the hero climbing up floating mountains to tame a flying dragon and claim one is a rite of passage in getting accepted into their community.