The two main themes I've read so far in this thread seem to be, a) As a good but not incredible film, why does Avatar get its own land (the OP question)? and b) How does this film do close to $3billion in tickets and yet leave no cultural footprint?
In a way, they share the same answer. Why does Avatar get its own land in AK? Disney (in the corporeal form of Bob Iger) went to Cameron to pitch putting an Avatar attraction into Hollywood Studios. So, like a ride or a show or something. Cameron comes back and says, "Even better, give me 15 acres and a half billion dollar budget and let me build you a completely immersive experience." This is an easy call for Disney. Cameron excels at world building. Think Terminator (the first two), Aliens, The Abyss, as well as Avatar.
What's in it for James Cameron? He gets another way to tell a story. A medium that is unique to Disney, though other venues try, is taking a story and putting the guest in the middle of it. (don't bring up Universal Studios. Been there, fun to be had but in this regard it's crap. You will never lose yourself to the story at US.) And Disney is the stage with an audience so large and so packed they can actively chase some away.
How does this answer question b? Avatar did not leave a cultural footprint because Cameron owned near every part of the movie (directed, produced, filmed, edited, and to a large part marketed) and Cameron makes movies to tell stories. Marketing the movie was really subdued for as big of a deal as it became. 2 or 3 cross marketting campaigns (Coke and McDonalds I think) and a few comicon panels, but mostly marketing for Avatar consisted of showing bits of the movie and letting word spread. He didn't write in a bunch of gimmicks or catch phrases because he didn't need to. Once the movie took off, there was no need to milk it with an endless supply of derivative works (toys, tv series, children's cartoon show, etc.). Consider Titanic (the movie, obviously) what of that film has endured as a cultural staple? The Celine Dion song? It would have been a hit without the movie. The "I'm king of the world" line, with your arms out?
Last point to quickly make (I know I ramble)... It's worth pointing out that more than two-thirds of the gross was earned outside of the US and Canada. The movies explicitly environmentalist message gets it dismissed by a huge portion of the American people in ways that do not happen in other parts of the world. And foreign tourists make up about 20% of WDW guests. I imagine a great many Brazilians will be contemplating whether to alot an extra day for Pandora Land or stick to the plan of wearing a thong all day at Blizzard Beach.