I watched the digitally remastered directors cut, and William Friedkin (director) gave a short introduction. He said that what you take TO the film is what you get out of it, if you believe that the world is a dark place you will have this sentiment reinforced. If you believe that there are forces that are fighting the evil in the world you will take heart.
I would like to discuss the reasons for this film's longevity. Why do you think that the movie is still popular?
I think that the film is eternal due to several important factors.
LW touched on its complexity (over at the Realm) and that's a huge factor.
I also think that our dogma is a prime factor. Of all the supernatural beliefs this film used the most common ones. I come from a religious background and though I am now an atheist and no longer believe in spirits or souls, the imagery and the power of the film was not lost on me. What I am trying to say is that most of us have been exposed to religious beliefs, so supernatural events that are a part of religion are easier to swallow (even if we don't believe it).
The film had great dramatic effect. It rarely went overboard in its portrayal of possession. It is as low-key as possible, vulgarity is only used in the "fit" scenes. This provides a stark contrast with the otherwise gentle world the film portrays. It's use of calm-to-excitement is perfect.
I also liked that the film did not fall prey to the use of gimmicks. there was one where the girl spins her head 180 degrees, but that was the only one I could remember. It was subtle, preferring to leave explanations aside.
There is only one real complaint that I have with the film. Exorcisms are performed with "the laying on of hands" and the film did not give much importance to the value of physical contact which is an inexorable part of exorcism rituals. I have witnessed many "exorcisms", they were not nearly as dramatic and in none of the cases did the "possessed" show any signs of possession. I grew up in a cult that routinely branded persons as having a "bad spirit". Their belief was that believers can not be possessed but can be "hindered". In other words the "bad spirit" would not enter the person, but rather hover around and follow.
I was considered a bad kid (which led to my expulsion from the cult) and was routinely subject to sessions in which "evil spirits" were "cast out". None of the sessions were dramatic and in most cases I was bored to death.
Even though I am now an atheist I still find that films about religion are easier to swallow that alien attacks, etc.
I loved the film, but it hasn't changed my thoughts or beliefs.