As an author, i will tell ya, it is really really difficult to come up with new and original stuff. I mean, before i even saw Avatar i had written a story about some primitive but spiritual people on a far away planet who were being exploited for a medical resource by a big bad company and it was up to teh hero, a protagonist who had been infected with a virus that was slwoly changing him into one of teh natives of teh planet, to rally them in defense of thier world in an epic battle. The most delicious irony is that i had just finished it the day before i went to see Avatar. *shakes fist* damn it cameron, you're lucky i liked your blue people or i would be pissed right off!
My point is, alot of good ideas have already been used before, to the point that it is nearly impossible to find or create somehting that doesn't come across as a rip off at some point or another. The challenge then, is not to worry so much about it, but to focus on how the story is told. Lord of the Rings for example, and Avatar, pretty simple story lines, but they were told in such a way that it became more than the sum of thier parts, it became an enjoyable experience. So, rather than focussing on "who had the idea first" lets try focussing on the movies own merits. Avatar was great becasue of its presentation, not just the stunning visuals, but becasue the enviroment and the universe it created was beautiful and immersive. Even though the characters were cut and pasted stock characters, they worked and were likeable.
I do wish people would stop hating on Avatar just becasue it is popular though. I am sure some people actaully don't like it, but i am sure many more are just 'hating it' becasue they like the idea of being in the 'cool' crowd. If they paid attention to the movie more closely, they would find that a lot of thier complaints are actually not really that valid.
Although it is frustrating, there are so many little things that the movie could have done that would have made it a whole lot better. Oh well, here's hoping that the next one will be attending the Dark Knight School of Sequelage.