It was trying pretty hard not to be science fiction, though. Its characters, you may have noticed, were pretty weak. And its plot definitely didn't need to be science fiction to get its message across; numerous people have noted that the plots of Avatar, Dances with Wolves, and Disney's Pocahontas seem to have been created by filling in the same Mad Libs sheet.
So what is it that makes Avatar science fiction? In short, the "avatar" concept. By including this concept in the film, James Cameron created a reality that asks the question "What makes us human?" And it asks the question really, really well.
