I cannot fathom anyone being scared of the Blair Witch Project. It was so disappointing to me. I was like...omg...are you fucking serious?
Things that contributed to how scary it was:
1. I thought it was real when I saw it. ( I know I know.. I was a kid.

)
2. It felt real to me.
I've always been a big fan of ARGs (Alternate reality games), ever since that movie. When I first watched it, the whole time I complained. "Follow the river! Stop stopping! Grr!!" The dolls set the tone a bit, and I found the forest chase scene a bit scary.. but nothing was truly terrifying until the house. You can't watch it like some punk walking into a house.. *you're* walking into the house.
The small tiny hand prints on the walls.. They made my skin crawl. And, the panic right as she was hit in the head and fell-a bit scary.. But the guy. That large guy, totally competent and capable at one point in time, standing obediently in the corner like a child punished by their parents. The imagination it took to imagine something dastardly enough to get that guy to stand there without even flinching--like he'd already long given up on life by then--it was so scary. They never showed this witch.. they left it entirely up to you. That information, thats all you have to go on.
It goes against everything in human nature, to stand there the way that man was. That.. concept. Was terrifying to me.

Maybe I put too much into it all.
I wonder if what scares us is very personal, or some people are more easily absorbed into things that obviously aren't real.
This may be true as well. I don't need something to be real at all for it to be scary--I tend to empathize with characters in films and shows very easily, and I can place myself right into their shoes most of the time. I felt Paranormal activity was a pretty scary movie as well, being as I believe in demons existing.. whereas someone who doesnt believe in the paranormal might have found the entire thing boring shadows and baby powder.
I should've also put Scary Movie on the list. It was scary because people thought it was funny.

That movie was pretty scary..