You make a good point, but I still think the horror genre gets overlooked when great movies overall are being discussed. How often are horror movies included on lists of “greatest films of all time” compiled by critics or film institutes? How often do they receive awards given out by critics in main categories, not just in their respective genres? Those are few and far between.

One of the greatest screen performances ever!

Touché! Part of its impact was indeed the fact that it shifted the expectations of audiences with regard to what “evil” in horror movies should be or what they should be afraid of. Up until then, it was mostly boogeymen, faceless monsters, and supernatural creatures. It paved the way for a new kind of horror villain.

I’d consider it a cross-genre horror, with elements of mystery, thriller, and suspense. Besides, it’s based on a novel that won the Bram Stoker Award given out by the Horror Writers Association.