As a kid I was interested in The Exorcist due to its cultural status, critical acclaim and, this is very important, the gross-out details. Specifically, I read the Ebert review as a kid and it mentioned vomit, and I wanted to know why there was vomit in a critically acclaimed movie. So I saw it and was shocked as a kid by some of the more brutal stuff, like Regan stabbing herself in the coot with a crucifix (okay, that example is still pretty shocking.)
As an adult, I can still watch the film but am no longer terribly shocked or grossed out by most of the shock-porno stuff and some of the exorcism is even cheesy (why does a stupid cut suddenly rip itself into her leg?) In fact, what I find interesting about it the most is, tying into Friedkin's Best Picture winner from two years earlier (French Connection) is the bleak, cynical, early 70s, "everything's going to hell" worldview (and The Exorcist is perhaps the most-watched film to exhibit that attitude, no? Maybe Taxi Driver comes to mind, but Exorcist is still more watched.) Like the dingy trashbag-strewn neighborhood Karras' mother lives in, or that Regan's mom is filming "the Walt Disney version of the Ho Chi Minh story," or that divorce was such a big deal, or the priest who has a drink, or "could you help an old altar boy out father?" Or Regan peeing on the carpet in front of that astronaut!
Of the horrific aspects, I will admit that the use of subliminal messaging is still a bit of interest, not just the satanic face flashing and that X-ray scene that supposedly made people throw up (okay, that scene wouldn't be that effective without the rumors) but also the backwards voice and pig-slaughter noises. The intro with Max von Sydow and the evil statue works pretty well IMO too. Actually my pick for the best scene in the movie is Karras' dream sequence where his mother disappears into that underground pass on the highway, where you see the satanic face flashing and the cinematography is all washed out and the sound is all dreamlike and inaudible.
And the biggest jump in the movie comes from the phone ringing too loudly when Karras is listening to Regan's tapes.