It's actually quite hard to spoil Avatar, because the entire plot is laid bare by the trailer, and besides, you've seen films with this plot many times before. One of Cameron's particular strengths is the reworking of well-worn tropes. This film has no more plot than one of those old IMAX 3d nature shows, and nothing remotely surprising happens at any point during the film. One exception; if you haven't watched any preview of the film, or the trailer, or read anything about the film, then Cameron does a neat job of introducing Sully several minutes before showing that he can't walk.
There are also a couple of points where you could, I think, be surprised by the plot if you had never ever seen a film before. When Smurfette explains that only five people have ever ridden Big Badass Dragon Thing, you would have to be a very naive filmgoer not to observe Chekhov's gun; in a not statistical sample, the twelve-year old knew Sully would turn into a dragonrider of Perndora, and the nine-year-old didn't. Plus, after seeing a billion movies where she walks out of the flaming ruins with muddy clothes, a big gun and a bad attitude, I was personally pretty surprised when Sigourney Weaver snuffed it in this one. But cranky elderly scientist with heart of gold normally dies, so I shouldn't have been.
So I think we can safely assume that Cameron was not intending to amaze us with twists and turns, and this is not a plot driven movie. Despite the lengthy battle scenes, it's not really an action movie either. It is certainly a travelogue; much of it would have benefitted from a soothing voiceover by David Attenborough. It is a polemic; we are told that Earth in 2154 has no greenery and are expected to deduce that it's a dystopia. Personally I doubt that a society that can grow genetically modified vat people can't manage to grow trees so I think we will assume that's handwavium. And there is something very dodgy about bringing all the technology of the world and the GDP of a small nation together to make a film about the simple life and the perils of over-development. I found the environmental message crude; the weakest thing about the film.
The 3d cinematography is, without doubt, the best that there has ever been. Cameron says, over and over, 'see, I know how to compose for 3d projection'. Reviews comment on how few obvious 3d shots there are; we get very few spears in the face. But in fact the obvious 3d shots are everywhere, if you know where to look; scenes shot through grubby windows, people enclosed by transparent monitors, lots of swooping. And there were very few of the sort of shot that doesn't work in 3d; this is the first 3d movie I've seen that doesn't have egregious window violations, or shots that just don't work in 3d. For example, there are very few facial closeups; most conversations happen at a 3d comfortable distance. Where window violations are unavoidable, as during the mass Hum-and-Sway, Cameron is careful to light the scene to move your eyes away from the edges. Even noted 3d haters, the twenty-first century equivalents of the critics who thought that adding colour to movies was inherently pointless, have largely been silenced by Avatar.
But the reviews have completely ignored what I think is the philosophical core of Avatar. We know, because we have read the title, that the main characters project into vat-built 'avatar' semi-clones. The film is very interested in the relationship between people and their avatars; there are three main characters with avatars and each of these relationships is quite different. But there is more. The Na'vi people ride land and air creatures (who have helpfully evolved with reins and stirrups in a hammer blow for Intelligent Design) by forming at least two different types of neural bond with them. They also have a less structured bond with their living planet. And the humans operate remote vehicles and man exoskeletons. Almost all of the detail of character development arises from one or more of these relationships. The main character is one half of a set of identical twins, all the better to explore individuality and genetic determinism with. The film is questioning the location of the soul, the nature of identity, and the difference between a person and an alien. And it does it pretty well.
Overall Avatar is fine entertainment, and provides enough to think about for an animated discussion over a meal afterwards. In our case, that meal would be a nice breakfast at Giraffe, as the only way we could see this film at the IMAX before New Year was to go see at at 6am, involving a considerable degree of inconvenience. Worth it though. I worry that it's too effects driven to stand the test of time as a movie to watch, but I suspect it will be a landmark in the development of cinema, for 3d rather than CGI and motion capture.