Well, did we expect any lesser treatment for the highest-grossing movie of all-time? And though it might not be justified as to why so many people set out to brand Avatar "uncool," it makes sense that this movie was met with a backlash, given that such a large portion of the Earth's population were lured into seeing it - how could they, after all, be the only person on the planet to have not witnessed this modern day Titantic? Opinions on the movie were split, of course, but it wasn't 'til a few months later when Avatar's good reputation began dissipate. People were p*ssed. The first cries of "Avatar is rubbish" began when people noticed similarities between the plots of movies like Pocahontas and Dances With Wolves and James Cameron's blockbuster. This "revelation" (if you can call it that) snowballed until the the majority of people who had seen Avatar were bullied into pulling a 360 on their original opinions, retrospectively forcing themselves to go back and re-evaluate a movie that a select few had decided wasn't as good as we'd all been claiming. It's a strange way to approach a movie you enjoyed; the bandwagon reigns supreme.
