Movies made in that era had different generalized pacing styles, and the technique of capturing things in a "scary" way has changed and evolved since then. So when you watch it, you have to put yourself in a different mindframe, which is hard to adapt to unless you've seen a lot of 60's/70's movies, like I have. My mom always would tell me about how everyone thought Jaws was the freakiest shit anyone'd ever seen when it came out, but now it looks like total cheese.
It's like watching The Bad Seed, a semi-horror-suspense made in 1956. If you watch it without the right mindset, it's laugh-out-loud silly. But with an understanding of film techniques from the 50's, you can get into it more, and appreciate it for what it was when it was released.
Now that being said, I didn't find the Exorcist very scary, but I can appreciate the direction and cinematography laid out for it. I wouldn't say it's a bad movie, but I do wonder why it's gotten all the hype it has over the years, right mindframe or none.
