I wasn't sure what to expect except that friends recommended it highly, and Cameron's movies have been entertaining.
It was an experience. As someone else said, the humans occupying their avatars in the movie is paralleled by the audience "occupying" the milieu of the movie via the 3D technique. Though the "experience" is unique and thrilling in itself, Cameron is a great storyteller, and without a good story, this movie would not be as memorable as it is. The story (though not particularly innovative), along with the characters, will touch your heart. This film is an order of magnitude advance in the technology of moviemaking.