Most people are brainwashed into thinking The Exorcist is scary and makes you think, when I find it (along with a majority of my friends) to be a pretentious bore that takes itself way too seriously, and to interrupt the boredom, there is an occasional shock cut of something “disturbing,” although I would describe them as unamusingly disgusting material that tries to make you feel dirty for enjoying it because it pretends to be so high-minded and that we should feel ashamed about our possible entertainment on such a “serious” subject.
What results is, boredom for a good hour, then every five minutes something “horrifying” happens, which doesn’t result in shock (as intended), but a kind of exploitative unpleasantness. If director William Friedkin had stuck with the idea of it being down and dirty the whole time, perhaps the film wouldn’t be so consistently unsatisfying and poorly paced.
If the movie had been directed by Brian De Palma in his heyday, it would have been what the book really is, which is black comic trash (even if it won’t acknowledge itself as such). In case you can’t tell, that would have been a good thing.
The Exorcist is pretentious because it takes what should be treated as trash (and really is) and pretends it is holier than thou. The trash level is revealed in the novel which revels in atrocities, and the 2nd hour of the movie which is what it devolves into trying to shock the audience, presenting humorous things, without humor.
What makes The Exorcist pretentious is that it believes it is something that it is not. It believes it is an important movie about faith and redemption (really, it’s just a flimsy and offensive criticism of single motherhood, in other words, if Burstyn had a husband around, this would have never happened) and the power of religion to overcome all of the real problems in your life. However, it is still just poorly disguised trash, treated as gospel.