I think that the purpose of Cameron was to present a beautiful planet and then show how mankind is capable of destroying it, much like our own planet and awaken some sense of feeling in most people if not everyone. With this point of view I think the film was brilliant because it isn't everyday that a film touches its viewers on a sentimental level especially action films.It was a bit depressing, what with the huge amounts of destruction of beautiful nature, and deaths of the gathered native force.
The basic story of Avatar has been seen multiple times before. Heck, Costner's Dances With Wolves is the same basic premise. Avatar may be a nice looking story but it is certainly not an original one.As far as standing the test of time, Titanic did it. Why can't Avatar? Maybe the storyline isn't Oscar worthy, but to be good Science Fiction, it has to be beautiful, and it has to blow your mind.
I think Avatar did both of those, and did them well.