The thing that was most jarring to me about Avatar is something that will probably stand out like a sore thumb to any other SF geek: The aliens themselves.
Look around our planet. Notice something about all the big critters, the vertebrates, with which we share it? Kind of a backbone thingy with a noggin at one end and four limbs of some sort? Sensory and intake openings on the noggin thingy end and a poop chute at the other? Notice how we don't have any six-legged lions with nostrils on their back and four eyes in their butt?
Now go look at all the big critters in Avatar: They're all hexapedal, or six-limbed. They tend to multiple visual organs. Their breathing orifices, separate from their mouths, are located low and forward in their torsos...
Not the alien people, the Na'Vi, however. They're just big blue humans with tails and slightly feline features. Further, given the extremely alien biology, we don't know how most of the other creatures reproduce: We know that the flying thingies lay eggs, since we see their rookery, but everything else may use budding or something equally exotic, for all we know. The Na'Vi, on the other hand, are rather clearly and emphatically... er... mammalian. And coincidentally have roughly the same modesty taboos you'd find on any beach in America.
This is xenology a la Roddenberry as expanded by Straczynski, wherein the universe is populated by humans who talk funny and can be differentiated from each other by the oddly-shaped bumps on their noggins or the way they pluck their eyebrows. Why the mercenaries of Western ExploiterCo Inc. LLC had to send an infiltrator to learn the aliens' ways and culture was beyond me; their ways and culture seemed less exotic to this whitebread suburban American than, say, the customs of the modern Saudis or Japanese. Egalitarian, monogamous, recyclers who love Gaia... how alien is that? These people work at my local organic grocer's. I've met more foreign subcultures in midtown Atlanta.
Too much thought went into the xenology here to leave this thread untied. Cameron's threatening a sequel. I'm hoping there's an explanation.