I'm fairly confident about this...partly as a fan of Cameron, but also because story/plot, although fundamentally important, need not be so complex or original to be good. As a quick tongue-in-cheek guide to previous work of his:
The Terminator: Robot from the future menaces woman from the present to protect own existence, future career as CA governor.
Aliens: return to th planet of the Aliens. Even with bigger guns, they are still dangerous.
The Abyss: Undersea explorers find intelligent life at bottom of sea, panic.
Terminator 2: Robot from the future menaces woman from the present to protect own existence, future governor of CA switches sides.
True Lies: Future CA governor is mild-mannered computer salesman by day, fights terrorists by night (should be familiar life situation to many HN readers).
Titanic: pretty heiress enjoys shipboard romance with handsome peasant until angry fiance catches her. Boat sinks, panic ensues.
Seriously, the fact that you can summarize a story easily tells us little about the process of working it out on screen for an audience. So while Avatar can be summed up as 'greedy humans want space oil buried under village of fighting smurfs, conflict ensues', the real story (as with most of his work) is what happens when an individual's experience of the world places them in conflict with the status quo, perhaps one that may not even exist yet. Critics may deride this as formulaic, but Cameron's main theme is the conflict between the individual and the social, which does not yield to simple analyses of good vs. evil.