So… are there any big fan/cult followings out there? You know—people who treat the film like it's less a movie and more a religious text brought down from the heavens?
Um, yes. Anything you can think of, it's out there. Blogs, film appreciation sites, fan fiction, cosplay, Tumblrs. There's probably also an underground amusement park in the works.
Part of the reason this movie has so many fans is that people want to be Marty McFly. They want to skateboard while hanging onto the back of a police cruiser. They want to make the bad guys look stupid. They want to be able to play the guitar like a boss. Any time you can make legions of people wish they could plug themselves directly into an adventure story, you're going to spawn a bajillion fan clubs.