Slender is unnerving and startling, but it's like porn: it gets a very specific, natural reaction out of you and that's that. And maybe that's horror to a lot of people, I don't judge, I personally prefer it if there's something deeper down than simply eerie darkness and some bloke appearing suddenly with a loud noise going OOGABOOGABOOGA Stupid Dog.
Amnesia has much more work put into it wrt storytelling and environments. It may be down to the same formula as Slender, but at least it gives you a reason to explore and continue other than being just a heart attack simulator in which you look for random pages in some random ass area.
I'm interested in the many upcoming FPS horror games, but if they can keep it fresh and interesting and not just dive back down to Slender's here-let-me-give-you-a-heart-attack trick to making a 'horror' game.