‘Tis the season…
I remember seeing this one in the theatres when it was released in 1978. My friend Leslie spent about half of the film in my lap every time a scene climaxed. To this day it’s still the most memorable horror film I’ve ever seen. Something about the low budget crude filming of it gives it an extra edge.
Fearsome Beard 
It’s just that feel of low budget and crude horror that give a good horror film that eerie scary feeling. The Exorcist is much the same for me. It’s what makes a good horror film to me.
John Carpenter knew how to terrify the audience. I can remember being scared shitless.
I recently heard a podcast on seminal horror movies and this was one of them.