We have eight legged spiders, six legged insects, etc... It's just on our planet that the ratio of 4-legged animals to multiple-legged animals is pretty high. Maybe that whole evolutionary branch of 4-legged animals on Pandora just got eaten up by all those 6-legged freak creatures. And the Na'vi survived because, well, they learned how to throw spears.
Or they have leftovers of the other two legs in their pants. And they get cut off in a weird cruel irrational religious ceremony just after birth.
Yeah, but again, the ancestral divergence between humans and arachnids is MUCH more significant than humans and other mammals, or even reptiles in this case. If memory serves, spiders are descended from some kind of cambrian period arthropod.
Hell, the eight legged things is the least of the differences. Spiders don't even have skeletons.
The point most of you appear to be missing is that the Na'vi are so outwardly similar to most of the other species we saw, but there are a few pretty fundamental differences that makes it look like something screwy is going on.
It'd be like if there exists on Earth a species that looks just like a centaur. There's just no logical place for it on the evolutionary tree. Ironically the creationists would love such a discovery.