Here are a few of the differences from the film currently in theaters:
– Earth and its environmental problems are explored
– We see Josh Sully’s Avatar being born
– It’s revealed the Avatar program exists to train Na’vi to be an indigenous workforce for the Corporation, since it’s so expensive to send human workers
– There are more humans, including a bioethics officer on the take, a video journalist, a head of the Avatar program and a second military dickwad
– There is an Avatar controller who is burnt out because his Avatar died with him in it. He committed Avatar suicide because he had fallen in love with a Na’vi girl who had been killed by the military
Head over to CHUD.com for a TON more detail and analysis. Be forewarned, if you’re one of the few who hasn’t seen the film yet, there are massive spoilers in that article.