The film is set in 2154 on the hostile world of Pandora where a Terran outpost seeks to mine a rare mineral resource with a silly name - but must first persuade an intelligent native race to abandon their homes above the deposit. They send agents in amongst the Na'avi in specially grown Na'avi bodies - avatars - both to persuade the natives to cooperate and to gather intelligence.
Avatar as a narrative takes in several somewhat familiar plots: conflict over resources or territory; double agents with divided loyalty; the outsider leading the tribe against his own side; Romeo and Juliet's love affair. The plot is simplistic, it doesn't really surprise or twist much, but it doesn't collapse or lose momentum either. The messages are pretty basic - pro-conservation, strongly evoking the plight of the rainforests, and anti-war. There is one point where the anti-war message is hammered home using contemporary references such as "shock and awe" in the dialogue but this is the exception.
It's fair to say then that Avatar draws heavily and openly on other films and books, for example re-using the iconic Apocalypse Now shot of helicopters taking off in front of a giant sun. Other influences include everything Anne McCaffrey ever wrote, as well as Dances with Wolves, Lawrence of Arabia, Braveheart, Pocahontas, Dune and so on.
Avatar as a performance: Sam Worthington as Jake Sully and Zoe Saldana as Neytiri are faultless in motion capture, and Worthington's live portrayal of a paraplegic soldier is also great. In support, it's nice to see Sigourney back in a sci-fi role and she is as captivating to watch as ever; Michelle Rodriguez is amazing too - would someone please cast her in the lead soon? She seems to be making a career of fine support work but is capable of much more.
Avatar as a spectacle: on a few occasions the 3D shots don't quite capture the viewer - but when they do work they are simply breathtaking. The ten years of world building has paid off and Avatar really is worth seeing for the spectacle alone.