Exotic alien worlds, exotic alien races, big action setpieces - this is clearly the movie a ten-year old me would have made if given a lot of money and writing/directorial skills by a magic wand.
Supposedly a lot of Cameron's inspiration was the "John Carter of Mars" series that he loved when he was a kid.
Avatar successfully visualizes a future where people like us travel to exotic, other inhabited worlds in a way that's adventurous and exciting - something that Star Trek did for some of us in the 1960s, but which it has been unable to do plausibly for almost four decades.