Honestly I wasn’t freaked out that much by The Exorcist when I saw it... though that could be that I knew exactly what I was expecting before I went in.
IIRC, I read the book before seeing the movie.
I also had read the book and knew what to expect before I saw it in the theater. Not that scary for me.
What made a lot of viewers uncomfortable and scared was that the original 73 release used subliminal images interspersed through the scenes. Death mask, other disturbing images flashed so quickly that the eyes did not register the images but the brain did.
See the following: https://www.bing.com/images/search?q=subliminal+images+in+the+exorcist&qpvt=subliminal+images+in+the+exorcist&FORM=IGRE
It was the first movie I saw in college. Went with a bunch of friends so had a blast.