Yes, the story is tired and weak. Pocahontas, Dances with Wolves, or "Fern Gully on steroids", are all reasonable comparisons. There is even some recycled material from the Aliens series,
However, none of that matters when the lights dim. The immersive 3D places you inside this beautiful fantasy world in a way that 2D cannot. Other 3D movies use the medium as a gimmick, a way to startle the audience with things flying out of the screen. Cameron folds the technology neatly into the whole visual package in a way that enhances the movie, not just showcases the technology.
Go see it. If you leave your expectations about story home, and leave your mind open to a new world, you will enjoy it.