So I'd like to take you on a Jungian exploration of the film so you can see the archetypal depth of meaning there, and know why so many people are drawn to the story.
Carl Jung's work with fairy tales indicates that they are the bare bones of archetypal stories, and symbolic of an archetype's energies and purpose. Archetypes are known through their symbols and the stories those symbols create - such as myths, and later when Christianity banned the old myths, the fairy tales they became, passed on through the generations, stories about how to deal with life.
The beginning of a fairy tale always sets up the initial situation in life: A king and queen long for a child indicates that the collective culture (the king and queen) cannot produce new life. Without new life and energy, a culture dies. The beginning of the story tells us where the problem or the wound is.
At the beginning of Avatar, we hear and see Jake, our wounded hero. His scientist twin brother has died and he is enticed to take his place on Pandora to make enough money to get his legs working. As he watches his brother's body incinerate, he thinks: He was the brains and now he's gone. Jake wonders if he'll be able to finish his brother's work, since Jake is the brawn of the pair. Jake was a Marine who sees himself as a grunt doing what needs doing, and now he is left with only his wounded body. He is paralyzed, and has no standpoint.