Roger Egbert gets rather distressed at the end of his review, worrying about the numbness of contemporary audiences who need extreme horror to feel anything at all... Bless. Revulsion at video nasties seems to have been quite widespread.
Sensory overload is definitely part of it, but dynamics is even more important. This film is looong, the characters and situation built comprehensively before the action starts. Indeed, the tensest part of the film might be before the theatrics even begin. The mother-daughter relationship is so sweet that you start dreading the eventual manifestation of demonic influence -- a development that unfolds with excruciating slowness.
But what makes the film last isn't so much the scare-tactics as the characters and themes of bereavement, mental illness, loss-of-faith and loss-of-innocence, which all resonate quite powerfully (Ellen Burnstyn, Jason Miller and Linda Blair are magnificent). There is also the beginning's gnomic visual allusions to relativism and nihilism -- Father Merrin is a mysterious character throughout, but he seems to be battling with demons that have existed for the entire span of human history. The devil is trying to convince us we are animals, and in that desert in Nineveh Merrin faces a pagan statue looking down on two fighting wolves. It's dog-eat-dog out there, except when it isn't. Karras chooses to sacrifice himself in order to save Regan. When you throw the mumbo-jumbo away, The Exorcist asks only that we don't despair, and that in the face of evil we have the courage to do the upmost to save each other. That's where its true power lies.