The story was seriously bad. None of the "bad guys" had interesting personalities at all. All the best movies have characters that have a fan base on both sides IE Star wars with Vader and such. I don't find any general quatrich fansites out there, but plenty of people dressed in avatar costumes.
The plants and stuff were cool, but at the end of the movie you don't even notice the unique world at all. Other than the sky mountain battle scene and the parts where jake explores the forest, the whole unique world idea is pretty much lost.
The humans seemed to have put no effort into defeating the aliens at all. The general guy goes on and on saying how the forest will eat you alive and stuff you out dead and how everything is violent and ready to kill you. But then he decides to fly a giant plane, even though he knows the navi are most powerful in the sky. He chooses to fly it through the vortex, disabling all targeting systems as well. They also decide to send out foot soldiers into a jungle full of giant creatures.
I think james cameron tried to make it focus too much on "Hey look i can make a movie wanting you to betray your own race and i'll be the first to do that!" instead of "Lets make a good movie, with a good plot, good effects, and actually put thought into the development of it." He writes a script about 8 years ago and then just pulls it back out of his drawer and gives it over to the design team and makes Avatar.
I thought the movie graphically was an amazing feat. The concept of the alien planet was unique as well. The way the aliens' forest is linked and how they have direct connections to their god and the nature was unique as well. It was a decent movie, but a lot could have been better. I hope james cameron improves upon the flaws in the sequel, and yes he is making one.