Very little of what follows is a spoiler because you'll work most of it out for yourself in the first five minutes, leaving you with 2h35m of brain candy to absorb.
Avatar doesn't have a fresh idea in its pretty little head but its head is very pretty. If you've seen Tarzan (fantastic tree-hugging jungle escapades), Dances With Wolves (out-of-town boy goes native), Aliens (bone-headed military with technofetish hardware) and the work of Roger Dean then you've pretty well got it - but it joins these well-established dots very nicely, with not a single bad performance and nary an unconvincing special effect. Sigourney Weaver - well, naturally, excellent. The aforesaid hardware will appeal to anyone who grew up on Gerry Anderson. Even the bad guys are a little better rounded than in Aliens - the chief civilian would really rather not massacre innocents if he can possibly help it, and the chief jarhead has a job to do which, okay, he relishes a little too much.
The story really is engagingly naive and would have us forget every example from history of what happens when more and less technologically advanced peoples collide. Even in Dances with Wolves, Dunbar knows he's only checked the advance temporarily: he and his friends must head west. Anyone who thinks, at the end of this one, that the humans won't be back in far greater force is a fool. "Nuke the entire site from orbit; it's the only way to be sure," Sigourney once sagely uttered in an earlier Cameron movie. Nukes wouldn't be needed in this case, just masses and masses of weed killer.
Then there's the whole questionable morality of turning so totally upon your own people. I can understand disagreeing with them to the extent that you go and live somewhere else but a massacre of these proportions just isn't on. We've been told that one check on the power of the colonists is public opinion back home, but when word of this gets back to Earth, surely politicians will be elected on the sole mandate of shipping the weed killer to Pandora. And, fatally, it actually gives a bit of sympathy to the chief jarhead. "How does it feel to betray your own people?" are his not unreasonable dying words.
So, zero advance in science fictional story telling but astonishing advances in the visual medium of telling stories. Even without the 3D, the alien world would inspire awe and the 3D itself isn't intrusive. I could comfortably wear the 3D specs over my own glasses and everything on screen looks completely natural. There is no gratuitous waving-things-at-camera to remind the audience they're watching in 3D and you half - but only half - forget it's there.
Whether a story needs that kind of visual telling is another matter. This one doesn't. I would love to see Cameron's Ghosts of the Abyss, which is a factual 3D documentary filmed around the wreck of the Titanic. That would be worth the extra effort. As it is, the 3D is a tool but that's all. Technologically, anything that makes the user jump through one more hoop to achieve an effect is doomed to failure, even if that hoop is as simple as putting on a pair of special glasses. (The behind-the-scenes people may of course be jumping through no end of hoops - that doesn't matter.) 3D will have arrived when viewers can comfortably snuggle down in front of the TV equivalent and watch it with exactly as little effort as they can switch the TV on now.
The CGI effects blend seamlessly with the real actors, so you can see 12-foot blue skinned humanoids and human beings travelling in a futuristic helicopter without once spotting the joins. And yet, when I think back on it everything including the humans appears in my mind's eye as a Playstation-quality generated image. Strange.
And finally, a prayer. The marines are so obviously of the same ilk as the ones in Aliens that I could well believe this to be the same universe ... and therefore, please God let no idiot studio exec decide that what the world really wants is Aliens and/or/versus Predator and/or/versus Avatars.