I feel I should clarify where I'm coming from on this. I don't drive and I live in the middle of nowhere, so the internet is really the only socialization I get most of the time. If it was just that it was a bad movie, I could live with that. They've made bad movies before (Teacher's Pet for example). But the insanity of the fandom and how massive it is meant that for months after the film came out it was literally impossible to avoid or ignore it.
Making it worse is that when I tried to have an intelligent discussion about the film, they wouldn't hear it and everywhere I went I was basically branded the resident troll because - GASP! - I didn't like their precious movie. Even now, two years after the film was released. not once have I ever seen an intelligent, well-reasoned defense of it. Not once. All I see are logical fallacies and ad hominim attacks whenever anyone says they didn't like it, not just me. And these are the people Disney is trying to please. Do you get why that would be aggravating?
Regarding Avatar, writing and plot-wise, sure, it's not the greatest movie ever made, but the world of Pandora is so visually stunning that I can totally understand why Disney would want to translate that for the parks, even though they're mostly doing it to compete with Universal and Harry Potter. And the Avatar fandom isn't nearly as in-your-face about it, so there's that factor too.
And that brings up a very interesting point. Avatar got a very mixed reaction and no one really protested that. Why is it acceptable to not like one massively successful franchise but unacceptable to not like a different one? Because it's Disney? If that's their reasoning, then that is, frankly, stupid.