Well, I liked it. I went to see it twice. (Once with friends, once with my son.) The story is not new, not all of the scenes are new. (The one where the marine enters the Navi village is shot frame for frame from a half dozen Westerns, he is even struck on the face and chest the same way as the character is in the originals.)
The battle scenes aren't my cup of tea and I cannot example why, so far in the future, weapons are still using explosive projectiles (bullets and rockets). What? Everybody forgot about developing those plasma death rays?
I also cannot figure out why the Sigourney Weaver character smokes, forgodssakes, smokes cigarettes?? This is the future? This is a movie supposedly about the purity of the natural order? Way to go, tobacco lobby assclowns!
Here's my guess on the breasts. You got a planet with horse-like beasties and toothy-doglike beasties and that first big ugly thing, they all were apparently mammalian, so ...going to be some mammaries showing up on other critters including the Navi.
The sex thing, oops, the nerve-ending hook-up thingie, coming off the head IS a little strange. The horses have it AND the more reptilian flying dragons have it too. hmmmm. Very old linkage evolution-wise. (BTW there is a missing sex scene between what's her name and the marine that will be included in the DVD. whoopie. Making whoopie by litterally hooking up.)
Okay. One last thing. The mountains aren't the only things that float. Everything floats. All that "land" which you see is just swimming around on top of a liquid center. Floating mid-air like that party balloon the day after you buy it. Not enough lift to float away, just enough to keep it flying.
Next we tackle what gases make up the atmosphere? Not human breathable but thick enough to fly in?