I guess it all depends one what kind of horror films you cut your teeth on during those tender pre-teen years.
For me, the films I found scary were the classic Universal horror figures... especially Frankenstein and the Monster form Hell (I couldn't even LOOK at pictures of that Hell Monster in my Famous Monsters mags), the Dr. Phibes films, Asylum, Willard and Omega Man. Then came Exorcist, Jaws and The Omen... films that re-wrote the book of horror. Due to my age, I couldn't see The Exorcist until YEARS later, but the commericals really scared me in a profound way.
I was a film projectionist when those Halloween/Friday the 13th/Nightmare on Elm St splatter films started to hit. Frankly, I found them unimaginitive and boring. But I also realized that they weren't made for me. But seeing all of these films and their knockoffs (I worked at a B-movie theater that got ALL of them, no exception) sort of innoculated me from their scares.
Sadly enough, I'm still working at a movie theater (part-time to pay for my Sideshow habit) and we get all of the Final Destinations (graphic snuff films), pitiful remakes (House of Wax, The Fog, When A Stranger Callls) and the modern films films that involve some "killer" computer virus or "killer" e-mailer. I'm waiting for a film that has a "killer" cell phone! Now that will be original! This week we have "Stay Awake"... and I'm probably going to stay away.
Only a few horror films have interested me over the years... Hellraiser, John Carpenter's The Thing, Alien, The Ring. And some were fun because they were spoofs of a sort and that had a charm all to themselves like the Evil Dead films, Shaun of the Dead and Bubba Ho Tep. Having Bruce Campbell definately helps.
So even though horror is not the genre of choice for me... I am looking forward to seeing Slither... maybe it's Buffy/Firefly connection I have with Nathan Fillion.