How did these movies get lumped together? They are completely different in storyline, tone, and message. The only similarities are they are intended as mass entertainment.

On the subject of inspiration for Avatar it also owes quite a lot to both Dances with Wolves and Dune. It's certainly popular, but that's not the same as being great. Visually it's quite gorgeous, but the plot is hokey and predictable. One of the most blatant deus ex machina plot resolutions ever. But for all that, it is an entertaining popcorn movie, but it's not even remotely close to being one of the best movies of all time.

The Dark Knight is a better film, not just because the actors sell their characters better, rather than being subsumed by special effects. In addition the story is about morality and ethics when confronted with evil. Avatar has a much simpler good-versus-evil conflict, with the side that every character eventually choses completely obvious from the moment we are introduced to each.

James Cameron is on the cutting edge of special effects, but he plays it very safe when it comes to screenplays.