I'll tell you why I hate it, it has nothing to do with the hype of the fans (although the fans can be kinda weird) I hate it simply because I saw it in theaters and thought was really bad.
the movie looked great, amazing at times, im not talking about the 3D im talking about the planet, it looked really nice, the flying creatures were beautiful, the Na'vi looked really cool, visually its an amazing movie, and then you add the 3D and people were going crazy.
but then, the story, as a movie, is not original, is lame and cheesy, the movie starts off good, but then it turns into pocahontas and dances with wolves fast, for me it got boring, no amount of amazing effects could fix that, the story felt thin,
Another thing I really hated is how one dimensional and cheesy the bad guys were, they were ridiculous, specially the main bad guy, he was so over the top that everytime he was on screen i couldn't help but laugh, I think most of the human characters were very one dimensional, even the good ones, for a movie with so much going on visually, the 3D, for having such amazing visuals,
it had very flat everything else, story, characters.. I guess for me personally, the amazing visuals were not enough, because sometimes i felt bored, sometimes i laugh at the wrong things, and sometimes the movie made me mad of how much better the story could have been, and the only reason i hated the movie is how silly and ridiculous the bad guys were, Also the scene where the tree is destroyed and they have to leave, again, it was supposed to be powerful and sad but it was done so cheesy that it made me laugh
I don't hate the movie, is not a bad movie, but is not a great movie either, story wise i mean.
then you have people saying is one of the most amazing movies, that everything else doesn't compare, some people were even getting depression because they couldn't live in that world...
that's I think what made a lot of people hate it, the hype,
this is The Best review of why some people didn't like it, you might want to see, at least the first part of it, he does a really really good job at reviewing it
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https://www.youtube.com/watch?v=uJarz7BYnHA