I really enjoyed Avatar. Sorry some people take movies so seriously that they can't enjoy a visual spectacle for what it is.
All I know is that I wanted to go to Pandora after I saw the movie. I didn't care that the plot was a rehashed premise from countless other movies (bag on Avatar ripping the plot from Pocahontas; that was a shit Disney movie, and I'm sure many of you would rather watch Avatar over Pocahontas if you had to choose between the two).
There is plenty of cinema out there that tackles more complex plots/characters/scenarios, and that's awesome. But I certainly don't mind a mindless, visual smorgasbord like Avatar when they release.
As for Cameron? Guy can do whatever the hell he wants. He'll knock out all those Avatar sequels, then move on to something else, I'm sure. He may even give up on them if the sequels don't do as well as the original. But it's Cameron, and Avatar made a shit-ton of money, so I can't see there only being one sequel.