One: It was early enough in M. Night Shyamalan's career that his name wasn't yet synonymous with "twist ending".
Two: The best twist endings recast everything you've just seen in a different light, and ideally, it should make even more sense in the light of the twist. Shyamalan's The Sixth Sense did that, but Unbreakable did it even better -- when it hits, both viewer and protagonist are overwhelmed by the horrific implications of the revelation.
Three: The twist -- and I won't spoil it here, on the off chance that a reader of my journal hasn't seen it -- relies heavily on the tropes of the comic-book superhero genre. I am intimately familiar with those tropes; superhero comics are My Thing. Despite that, I did not see it coming. I was utterly gobsmacked.