Of course the movie only shows a handful of technological advancements over the present day, which makes its supposed setting, 140 years from now, pretty unrealistic. But there's the anthropic principle of popular movies to consider: the masses must be able to identify with (and mostly understand) what's going on, in order for a feature to be popular. There's a reason high concept 'hard' sci-fi novels (feasible imaginings of our future) are such a niche market, making a Hollywood budget film that emulates this literary genre would be financial suicide.
However, unlike most movies shoehorned into the sci-fi genre, Avatar appears to have been *very* carefully crafted to make the necessary 'anthropic compromise' plausible (for those who care to think about such technicalities; myself and you). Of course, if one understands the consequences of accelerating returns, you still just have to ignore the lunacy of the time frame; it's there to help suspend the disbelief of the ignorant majority.
Cameron has deliberately found context that justify the compromised details, working around some explicitly in dialogue and leaving hints about others lingering in the background details:
By limiting our experience to relatively small group of persons, all in a very unusual situation, it would be like aliens trying to assess the peak technological capability of contemporary humanity by sampling a single household. What if the sample lives in a mud-hut, or just an elderly couple (in the post industrial world) who never did get on with those newfangled televisual thingies.
This is why Ian M Bank's Culture novels have endured a couple of decades so well: because he makes it clear that the Culture is a massively complex, chaotic, heterogeneous society where the lifestyles of the citizens are mostly determined by their whims, there is no status-quo, or must have state of the art set of appliances that all the characters use.
This plot mechanism may well, I think, have been inspired by Vinge's fantasy of galactic 'Zones' (in “Fire on the Deep”): advanced technology fails/disintegrates in the deeper regions. Somewhat of a blunt instrument, it nonetheless allows one to avoid having to describe post-singularity technologies. Perhaps unseen, on Earth (in Cameron's imagined universe), there are anti-gravity hover-tanks that can descend from orbit, deck out with smart munitions, containing nano-scale/quantum circuitry, all of which are as useful as a brick on Pandora.
 The sensor rig-out in the coffins implies that brain signals are read by some kind of NMRi scanner and haptic/electrical feedback gives bodily sensations. This would be a paradox given the previous troubles with technology mentioned above: that such a high level of information processing and transmission would be achievable in a place where they're having to fly by eye.
Because the details here are never explained, I'd expect that there aren't brute force processes going on here at all: the equipment merely tunes/amplifies the individual's brain activity so that it syncs with that of the avatar. Some kind of quantum mechanical, spooky, action at a distance type of phenomena. Hence the need for a cloned brain (Jake's avatar was grown for his monozygotic twin).
This stinks like Roger Penrose's objectionable ideas on why strong AI might be impossible; ugly attempts to keep humans at the centre of creation. This just happens to pander towards a (Hollywood) Christian view of souls and religion (i.e. no coincidence I'm sure). Despite this, I am prepared to swallow it as a founding axiom for the fictional universe, to allow for the possibility of continued human strife, free from the care of God-like AI (As Star Wars hints is the case in the newer instalments). If anything, this caveat creates a hypothetical world that is shown to be undesirable. A Blade Runner esq. sprawl of cyber-punk slums, no doubt. Stuck pre-AI by some unspecified limiting factor.
 The human military are defeated in the end, but it is not a totally unrealistic fairy-tale of Disney forest animals trouncing an imperial army:
The military forces are a private security detail for a commercial mining operation. The soldiers are military wash outs or retired. Even the chief executive officer there is clearly an annoying idiot the board of directors wanted rid of, but couldn't fire. And one can bet funding for equipment and manpower wasn't nearly as much as Colonel Miles Quaritch would have liked.
However, even this sorry division of Dad's army with myriad technological handicaps was more than a match for the biggest coalition the Na'vi could muster. Jake's air attacks are repelled and the ground forces are being massacred, until mother Pandora (Eywa) steps in with an unstoppable wave of beasts.
This has major implications for what Eywa really is. A world brain of interconnected flora & fauna, holding a sea of dead souls for perpetuity. A camera friendly version of Reynold's “Pattern Jugglers”: an alien, ocean based mind/soul repository composed of prehensile seaweed, with very limited global agency.
If Vinge is *really* being ripped off, then it could be a that Alpha Cenauri (A or B, whichever Pandora's gas giant is in orbit around) was a rogue stella system that was travelling into the deep of the galaxy (until recently it was gravitationally captured by it's partner star). A convenient 'spacecraft' for a transhuman intelligence designed to run on resilient biological substrate, thus requiring the best part of a planet. As such Eywa's distributed memory could be a greeting a from transcendent civilisation outside the 'zone' (that Earth is in), *their* avatar/explorer of the deep. At any rate Eywa could hold the memories of other (alien) visitors from the deep past. A Pandora's chest indeed.
If Eywa is an interfering semi-god (as the beast uprising implies) then that could help explain how the Na'vi manage to live such an idealised version of pre-industrial life: their God is constantly tweaking their environment (and them) to keep them comfy and well adjusted; a caretaker.
Or then again, maybe the whole biosphere is supposed to be perfectly natural, and unobtainium is just highly compacted Na'vi bones and tears, like Soulstorm Brew in Abe's Exoddus.
It couldn't have hoped to have served as a prophecy of a real future, but it has equipped society with some visual props and examples to help it start understanding some of the issue it's going to be facing there. Even if those are examples held up by scientists, technologists and futurists for being wrong.
It's just unfortunate that Avatar's version of environmentalism looks so much like a technological rejectionist's wet dream of living 'in harmony with nature'. WALL-E did slightly better (despite it's civilisational homogeneity) in it's ultimate message, that one has to *deal with it*, not just run away. There is a happy medium between industrial/commercialism and the natural environment, and beyond that (fingers crossed) an even happier outcome when technological reach totally outstrips nature's scale.