The Exorcist/YMMV

The Exorcist

Adaptation Displacement: Not many people are aware that there was a novel first.
Adult Fear: Look at it from a parent's point of view. Your daughter is ill, there's nothing you can do about it and no doctors or medicine can help her.
Awesome Music: The perennially-creepy Tubular Bells.
Complete Monster: Pazuzu, of course.
Contractual Purity: The inverse happened to Linda Blair when she tried to break into romantic comedies.
Harsher in Hindsight: Father Dyer while entertaining the crowd at Chris' party, gives his interpretation of Heaven. Not only is the happy go lucky priest forced to give the Last Rites to his dying best friend Karras at the end; he's also violently killed by the Gemini Killer in the third movie, who has possessed his deceased friend.

Father Dyer: My idea of Heaven is a solid white nightclub, with me as a headliner for all eternity and they love me.

HSQ: Pretty much Karras' reaction when he finds Regan to be genuinely possessed is this trope.
Hype Aversion: The cover art declares it to be "The Scariest Movie Ever Made." That can definitely turn people away or lead to disappointed viewers.
Moral Event Horizon: If Pazuzu's possesion of Regan wasn't bad enough, when he posseses Karras at the end, he immediately moves forward to KILL Regan since he doesn't need her anymore. Bastard!
Memetic Mutation: Ever saw a screamer? If yes, there's a 99% chance demon-possessed Regan jumped in your face at least once.
- Or an image of Captain Howdy.
Narm: Most people find it scary, but quite a few find it downright hilarious mostly due to just how over the top it gets: a good example would be the restored spider-walk scene.
Nausea Fuel: Most people were physically ill and fled theaters during the medical scenes, especially the spinal tap.
Nightmare Retardant: The Exorcist in 5 Seconds
- Some find the effects work a bit outdated.
Squick

Ensemble Darkhorse: According to jabootu.net, a background prop they call the "Nut O' Fun."
Memetic Mutation: "I will spit a leopard."
Narm and plenty of it.
Sequelitis
So Bad It's Good
Sophomore Slump

Canon Discontinuity: In the third film, Fr. Morning disappears and reappears in the padded cell, chiefly due to the Executive Meddling described below, of adding the character post-production.
Misaimed Fandom: In general, this seems to be averted; most viewers tend to see the Gemini as a terrifying villain, nothing more. However, this was played very straight with one person: Real Life Serial Killer Jeffrey Dahmer. According to forensic psychiatrist, Park Dietz, Dahmer viewed the Gemini as something of a personal role model because of the power he was able to wield over his victims and captors. He also liked the character of Emperor Palpatine for the same reason.
Narm: It is NOT in the file! IT IS NOT!!
Surprisingly Improved Sequel: After the embarrassment that was Exorcist II, The Exorcist III with George C. Scott turned out to be a pretty damn good little horror movie; some think it's horribly underrated; and a few (admittedly very few) even go so far as to say that it's both better and scarier than the first movie.
Uncanny Valley: The animatronic grinning pianists at Grand Central during the Dream Sequence in Times Square. Deliberate for Nightmare Fuel.
The Woobie: Father Karras

Surprisingly Improved Sequel: Some people say that it's this when compared with Exorcist 2, 3, and The Beginning.