I have to say that I feel a little sympathetic to some of those that feel sad after watching the movie. Some, like me, had depression and an unhealthy dislike for humanity (as in, homo sapiens) before even seeing the movie. Pandora was a dream world, one I wish I could go to (and hopefully not get eaten...). The Na'Vi people were absolutely beautiful and rich in culture.
There's the idea of the avatar itself. I've often wished for a new body, over and over. The idea is extremely appealing to many people.
I also sympathize with the poster in the article that spoke about suicide. It's the little things that can drive someone over the edge. The feeling has happened before after many movies, for me at least.
Please don't judge them by the fact they wished they were somewhere else, wanted to be someone else, to live in another time.
. Cameron managed to pull emotion from a lot of moviegoers.