You may remember my answer from last night. I thought of another on this morning which I'd forgotten because it's so common most people don't realise it: humanoid aliens.
The Navi might have blue skin, but they have human faces, human-shaped bodies and human behaviour, right down to their mating rituals (Jake and Neytiri make out like humans do).
The film reasons for this are pretty obvious- the audience needs to be able to empathise with the aliens for the film to work. The aliens have to be, ultimately, humans in disguise. If you go the other way, you get Sigourney Weaver's aliens. Great for a particular type of story, but not this one.
But when you think about it- why are the Navi so similar to humans, given that they evolved on a planet over 4 light-years away? There are so many paths evolution could take that the chances of them being the exact same are astronomical. It would be like Alice thinking up 100 random numbers in Australia, and Bob thinking up the exact same numbers in the UK. Too much of a coincidence.
The story is similar for the trees and other organisms on Pandora, many of which have very terrestrial features, despite having nothing to do with Earth.
It's not physically impossible, but it's also very unlikely that the battle between the humans and Navi would have played out the way it did, at least if the humans had any semblance of logic. Let's see now:
* Cameron makes use of a technobabble "flux vortex" to negate the effects of autotargeting, so one does wonder why they flew straight into the vortex instead of around it.
* It's very strange that arrows go through the helicopter armour, when they glanced off them in the earlier battle without so much as a scratch.
* Helicopter gunships aren't designed to fight in-close like they did. Modern gunships at least prefer to fly around two miles away from their target and pick them off from a distance. (Flux vortex or not, radar targeting doesn't work at close range if the target moves fast enough). Again, it makes you wonder why they continued to fight at close range.
* The fastest birds on Earth have a diving speed of about 350 km per hour, but they can't sustain it- their flight speed is much lower. Modern helicopters cruise at around 270. In short, the humans should have left the Navi attackers for dead.
* The ground battle is also rather lopsided. Heavy machine guns that can penetrate plate steel can't kill animals? I don't think so.