Premise and plot.
In a movie of such scale it would be unreasonable to expect a truly unusual premise or complex plot -- it's believed (and most likely true) that a large enough fraction of potential viewers simply won't get it, and the movie will flop. Even without that, the amount of time and number of scenes that would have to be shot with all those special effects would be excessive, and no amount of popularity or expensive IMAX tickets would make up for it. The original Star Wars were already pushing the limit in 70's when movie production was cheaper and viewers were more tolerant, and The Matrix did the same in 90's. So I don't see much of a fault in having a relatively simple and somewhat anvilicious plot.
Jake, the protagonist, is a part of the Earth's invading force on Pandora, a resource-rich planet. Mining company (that mines aptly-named Unobtainium) is in the state of unholy meld with both military and researchers. Na'vi, alien natives of the planet, seem to be ignorant or at least unconcerned about the value of resources that Earthlings are after, yet none-too-happy about foreigners messing with them, so Earthlings are trying to use their scientific might to establish communication with natives using titular avatars -- artificial alien bodies remotely controlled by human minds -- yet run into a massive brick wall of cultural incompatibility on top of general distrust.
The protagonist, self-admittedly being a comparatively unsophisticated Marine, joins the program with the scientists, and through his willingness to participate in supposedly primitive culture gains mutual understanding and greater respect for the natives than for greedy mining company. Eventually he joins their side when trigger-happy military mounts a massive assault -- supposedly betraying both Na'vi (by providing information to the military) and Earthlings (by stopping the bulldozers and fighting on the Na'vi side). Scientists do not share his insight into the culture, however they have even better idea of what is being destroyed -- the natives' gods and habitats form a Solaris-like planet-size brain -- and are just as horrified by the massacre, so they have to make the same choice.
Protagonist manages to out-alien-warrior the alien warriors to unite them against the threat, and invaders are driven off. In the process, protagonist and scientists discover that despite primitive organization of society, native hunters-warriors ride animals that are a match for human military vehicles, and natives' ability to "link" their brains with each other, animals and gods/planet-size "brain", has the same capability as the computer-based technology used to drive avatar bodies.
Did I miss anything important (that is, everything that is not mandatory sex and battle scenes)? I don't think so. Being inclined to seek flaws in everything, I can mock the unoriginality by saying that this is "Lawrence of Pandora" and the moral of the story is "No blood for Unobtainium". It would be unfair -- if arrogant denouncement of foreign cultures and disregard for human (or alien) life in the name of profit were such a beaten cliche, we would not have our own governments, companies and whole societies engaged in exactly those things right now. The twist that aliens are not nearly as stupid as they seem, and that their "gods" are real and take better care of their planet than Earth governments did of Earth, is an important point, and something that turns this from "futuristic fantasy" into "science fiction", what is, I believe, the intended genre of the movie.
However there are other things that bother me in this plot. First and foremost, portrayal of natives is hopelessly in the "Noble Savage" cliche territory. Seriously, we are not in 19th century anymore, we can't characterize a whole race or species as "proud hunter-warriors living in harmony with nature, disinterested in modern doodads", and expect suspension of disbelief to cover up viewer's understanding that such a thing never existed, does not exist and never will. Na'vi have tribal culture, are more like hunters than warriors, and at very least at intuitive level show enough sophistication to use their links in their everyday life, communicating with animals and "gods". For all we know, they, or at least their shamans, may have a library of books on advanced math and science in there. Or spirits of their ancestors post pictures of lol-viperwolves on alien 4chan. Or maybe even their gods/spirits somehow managed to run the whole thing entirely on intuitive level, and their apparent disinterest in science and technology is keeping them from gaining better understanding of themselves and the world around them. There is nothing in the movie to answer that question.
Depiction of the life of hunters is ambiguous -- from the amount of training and apparent dangers it seems that they may be really an equivalent of early human hunters-gatherers who were getting killed right and left while barely supporting survival of the tribe. On the other hand, they just as well may be an equivalent of modern fishermen -- their job is more dangerous than most, however accidents are rare, and food they produce is sufficient to support a reasonably healthy lifestyle. If they are supposed to be the first, it makes a giant hole in the argument of them being morally superior to humans because this would mean a completely preventable disregard for the sentient life -- it would be strange if human scientists didn't have anything to offer in regard to, say, agriculture. If they are supposed to be the second, then humans are indeed wrong about their "primitive" nature, yet there is nothing in the movie, seen or said by human observers, that confirms that or provides a reference point. "Noble Savage" denies or glosses over this distinction, however as I said, we are not in 19th century anymore, and it's related to the most fundamental part of the movie's premise.
The whole situation with humans deciding "they don't need anything we have" (and keeping this belief to the end of the movie) shows that either hmuans of Na'vi are being willfully ignorant for no reason. The battle and consciousness transfer procedure show that in practical matters capabilities of humans and Na'vi are approximately evenly matched, however they use different means to achieve those capabilities, have different sets of knowledge, and see their place and role in the world in a completely different manner. Save for rabid xenophobia, there is no reason for both races not to exchange knowledge once battles and attempts of resource-pillage are over -- maybe then more humans would learn to like life on Pandora, and aliens would get greater understanding of things they already "see".
Unless, of course, gods already have the same knowledge yet are against either giving it to Na'vi, or letting Na'vi to learn it from humans in more humanoid-alien-understandable form. I guess, it's not humans' job to question what Solaris-style brain is doing with its own planet's population (especially after same humans attacked and damaged both), but forcing ignorance places him rather far into the "evil gods" territory. Unless the whole story is supposed to be about evils of science and technology -- what I don't really see in the movie because most of the humans' stupid and destructive actions were caused by unwarranted arrogance and ignorance. No, seriously, a movie made entirely using sophisticated technology, has to involve some respect for it, so I don't think, my own love of science and technology prevents me from seeing some kind of valid argument against it.
I am more bothered by similarity to this Stargate episode -- up to the ending with Earthlings expelled and natives' "gods" not providing otherwise achievable space travel capability.
Choosing to portray Na'vi society as a simple tribe with single political leader (chief) and spiritual/intellectual leader (shaman) is consistent with "Noble Savage" cliche, and can illustrate a valid point that not all "values" specific to the current human societies are universally desired, however it makes very little sense in the context of the movie. Such social systems develop when otherwise aggressive members of the tribe would cause more damage by infighting than massive concentration of power, and when overworked and malnourished commoners would rather defer thinking and communication with "gods" to the shaman, however Na'vi don't seem to have those problems. They may have this system as a relic of earlier times, or, more likely, author's imagination does not cover such matters, so using a cliche was the only way out.
Technical details and other nitpicking.
First of all, atmosphere density and gravity (if one has to believe http://james-camerons-avatar.wikia.c
Military seems to be suicidal when it comes to their machines design -- while civilian bulldozers are remotely operated and have cameras (that are their weak points), everything military features a cockpit in front, made mostly out of thin, fragile glass, that makes them look like WWII bombers and fighters. Not surprisingly, those mighty machines end up being easily defeated by pretty much anything heavier than an arrow hitting those things. I thought, someone already implemented a better solution for a homebuilt submarine -- and even kept a cockpit in its normal place. Yes, I understand the artistic value of emulating WWII weapons, but I thought, the scene of firebombing an alien equivalent of a city was enough of a reference already.
Transparent displays seen in the Earthlings' labs and offices everywhere are among the most impractical things that look cool in the movies. See this glare on your screen (or remember how it looked before anti-glare screens)? That's reflected light. Now imagine the light shining through your transparent screen from a window. Reflected from a glass of tea and a spoon on your table. Or from a golf club in the hands of a guy playing in the middle of the room in the movie. He even has an audacity to point it out that a technician who claimed to see his successful shot, looked at the monitor at the time. It's not enough that Star Trek and Tron inspired countless attempts to use giant touchscreen as the primary input device on a desktop computer (and so far produced nothing but pain in the fingertips and fatigue in arm muscles), now we should wait for HP (I think, it will be HP) to remove backlight from a transparent LCD, put a glass in its place, and market it as a new display device.
Personally I find the idea of such device offensive exactly in a way lampshaded by the office-golfing guy -- it assumes that what I will see through the screen -- "important" people walking around me, changes in the office scenery that I am supposed to react to -- is supposed to take precedence over concentrating on the work that I do on the screen. Thanks a lot. On the other hand, the aforementioned cockpits have HUD supposedly based on the same technology, that perform their function perfectly -- seeing an enemy in front of you usually does take priority over instrument reading. Of course, exterior cameras and enclosed cockpit would make such a thing moot anyway.
CGI production and other technology.
This is obviously a step forward in 3D movie production, and "eve candy" scenes are very well done. I only have one nitpick -- in many scenes Na'vi bodies' geometry does not match physics of their movements -- they seem to be rotoscoped actors rescaled to Na'vi geometry and not simulations, so they move as if they are shorter and heavier than they really are. There also may be less to the facial expressions than meets the eye -- from description of technology that I heard it is supposed to be human facial expressions recorded from a camera, converted to a 3D mesh of a human face, then human mesh is converted to a mesh of the alien face, and finally rendered. I think, what I see must be human face recorded as two 2D images, converted also in 2D based on a translation between projection of human and alien meshes taken from the same angle with the same position of a jaw, re-colored to look alien, and matched with an alien body. If the body is rotoscoped, camera angles will automatically match, so as long as scaling/morphing matches 3D meshes, actual 3D data from each frame is completely unnecessary (think of it as image being already bump-mapped). This can explain too human-looking skin on alien faces, and realistic movement of facial muscles. Also the geometry of alien face is actually very close to human -- nose is flat and wide in its upper part, eye sockets are not as deep, eyes are larger, and cheeks are flat, but that's it. I am rather skeptical about getting full 3D mapping of a face from two camera images, and transforming it cleanly between meshes.
Oh, and one more thing. Goddamn Papyrus font in the title and subtitles. What is this, Papyrus is the new MS Comic Sans? Did they just have to include something offensive to all geeks, so those geeks won't write long whining reviews like the one you just read?