The reason the aliens are humanoid is because humans are the intended audience.
Did you notice how the rest of the creatures on Pandora were all based around 6 legs, 4 eyes? Logically the Na'vi should have 6 legs and 4 eyes too, but that would make them too alien.
You want your aliens to be alien enough to seem alien, but not so alien that it's difficult to empathise with them, the director wants you to empathise with the aliens, to understand them, to feel sorry for them, the movie is supposed to tweak your emotions. It's extremely difficult to feel sorry for ravenous bugblatter beasts. Even District 9, with it's aliens that were essentially prawns, had to make them have a humanoid stance, mannerisms and eyes in order to let you empathise with them (and they did a truely supurb job).
The fact that the aliens are not very alien is not some sort of error or mistake on the part of the author, it's a restriction of the media, and it's deliberate, since the rest of the life on Pandora is cohesive, it's ovbious that the Na'vi were deliberately made humanoid due the fact that the intended audience is humans.
I really, really wish people would stop running around screaming that X sucks because the aliens aren't alien enough.
There's nothing wrong with the story, it's not a new story, but it's solid, implimented well and there are no major plotholes. The story is competant and polished rather nicely, if lacking in originality, but then again there are only about half a dozen unique story threads, so a lack of originality isn't surprising.
The visuals are stunning, absolutely stunning, i'm going to need to buy the DVD and watch it several times just so i can get the full idea of what's going on in some of the scenes; so much is happening, the backgrounds are beautiful, with the characters moving around purposefully and actually -doing- stuff instead of just sort of aimlessly ambling.
Avatar is beautiful; yes the combining alien and human DNA thing is rediculous, but that's why it's called fiction.