Ironic that you used these words. I've always found the animation to the be the weakest part of Avatar.
The textures are nice and the lighting is nice, but the animation always looks a bit off.
Braids are either incredibly heavy or they're made of pipe cleaners. Head-Flower has petals of super strength. Necklaces aren't affected by wind at all. Wind has no effect on her eyes or face, and she doesn't appear to have any special wind-protecting eyelids.
Tail flapping around like it has a mind of its own. Lifeless eyes that don't narrow or move to increase aim. She also has some weird feather/flower thing growing out of her right shoulder.
Eyelids don't scrunch up at all, even though the bridge of the nose indicates his whole face should be crinkled in agony. His hair also doesn't fall properly when he turns his head.
Lazily floating along before passing the tree, then sudden increase of speed to "slam" into the root. Water looks like jello in the foreground. Water speed also doesn't match the 'floating' speed or the 'slamming' speed.
It's rather telling, though, that such tiny details have to be focused on in order to see flaws. The movie is definitely the best CGI out there, but I can see why it doesn't quite look right to people.