So I know they were designed to be appealing to moviegoers. I think one review called them "familiar yet pleasingly exotic." I am not even sure what it means to be pleasingly exotic, but I think that hits the nail on the head. You feel really weird looking at these strange creatures and yet you find yourself strangely drawn in.
Non-sequitur: Why do tribal chiefs ever only seem to have daughters? Why don't they ever have sons in the movies?
I'm not really sure what does it for me - the super obvious resemblance to Native American and African tribes? The hugely expressive cat-eyes and human-like mouths? The way you can readily recognize the human actors who played them? The sensually exposed blue skin? Their raw physicality? Whatever it is, it's the stuff of dreams.
That said, when I got out of the IMAX theater tonight, I called Garry and said it gave me the creepin' willies to watch avatars make out, let alone have 'avatar sex.' I'm not sure why, but I think it has something to do with my feeling that such shameless displays of affection are specifically a figment of Western filmmaking, and these creatures seem decidedly non-Western.