HM, to me, is the perfect dark ride and is one of, if not the best dark rides in the world.
When I was a kid, every cheesy carnival haunted house always made me anxious and scared the crap out of me. I have always found HM to be kinda creepy, not scary, but fun at the same time. Also, if you look at the majority of Disney theme park attractions, they are light hearted, alot of princesses and classic animated characters. HM has a darker tone to it, a more sinister feel and story than almost everything else, and I think that appeals to alot of people. And, the technologies that the ride uses are classic but still as effective as they were 50 years ago. I've been on HM probably 20 times or more, and each time I always see a small detail that I've never noticed, which keeps it fresh for me.