As many here have already alluded to, its not about what the ride is based upon, its all about the execution of the vision and immersive environment, and how far out of reality that takes you in my opinion. Splash Mountain is based on a movie that many don't know of, fewer have seen, and I venture to say that almost no one particularly follow with 'cult like' eagerness. That aside, its frequently lauded as one of the best experience Disney has created to date. Similar for Tower of Terror, I never watched nor never cared for the twilight zone, however that ride puts me into an immersive environment that nothing in the real world can touch - and therefore I ride it again and again and love it. Furthermore, I hate hotels as I travel a good amount for work! Why are these rides great and why do they stand the test of time, because the execution and immersive environment is phenomenal.
All the Harry Potter stuff is great, because it does have a following, but what is the real environment that you are in? Quite honestly its a dream shift version of a london village where the buildings don't contain right angles, normal sight lines and architectural features. Don't get me wrong, its neat and interesting - but it doesn't take me all that far from the real world. So what if they did a star wars land, lets say the moon of Endor and Tantooine, ok great, again these are just small spins on real world places, one a dense forest and the other a desert outpost with some 'otherworldly' architecture and tech.
I saw Avatar, and I thought it was an entertaining movie, due in most part, to the CGI world that was created. Its unlike anything anyone on this board or great world has ever experienced. If Disney can translate that CGI immersion, complete with the scales, colors, textures, gravity deifying features, etc, then I would venture to say it will be a great success because anyone who walks through the gates will be transported to a place that is a COMPLETE diversion of reality and of what we know of this world.
Some of the greatest movies of all time wouldn't necessarily create great ride experiences and/or lands, otherwise Great Movie Ride in HS would require a fast pass to avoid a 120 min wait.
So lets all focus on what this land is really about, not the political message buried within this movie, or the lack cult following compared to Star Wars or HP, but the immersion and amount of reality defying detail and execution that imagineering will put into this land. I for one love Star Wars, I am 80's child and a Sci-Fi aficionado, but to be honest, Star Wars land wouldn't really do it for me - its just too aligned with the world in which we live with some additional technology; so I am glad they went with Pandora and I cannot wait to see what they come up with.