Science fiction is largely about telling one story while pretending to tell another.

In the case of Avatar, it's a couple stories as old as man (and perhaps older) - use of violence and cohesion of the powerful against the weak, impossible love, hero warrior enlightened and betraying his original allegiances and religious experiences are just the few that pop up after 2 minutes of thinking about it.

And, of course, you get a big reaction when the audience perceives it as telling their story while pretending to be about blue aboriginals of an alien moon.