These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Complete Monster: The foul-mouthed demon Pazuzu was the evil spirit that possessed Regan MacNeil in the film. He used her to commit his evil deeds such as lashing out at others and shouting obscenities such as the (in)famous Your Mom quote. When faced with the Catholic priests Merrin and Karras, he mockingly encouraged them to try to remove his influence from Regan. When Merrin was left in the room with the possessed girl, he has a heart attack, which was believed to have been caused by Pazuzu himself. This left Karras alone to fight the demon. When he successfully removed Pazuzu from Regan, Pazuzu immediately went into him and tried to force Karras to kill Regan since she was of no further use to him. Pazuzu is called the most frightening creature in a horror film for a reason.
Contractual Purity: The inverse happened to Linda Blair when she tried to break into romantic comedies.
Fridge Horror: The spider-walk scene in the book is less terrifying at first than completely, inhumanly alien. Blatty's description doesn't sink in for a minute. Then it hits you.
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!
There's also the fact that in the third film, he convinced the spirit of the late Gemini Killer to possess Karras's body and was responsible for countless human deaths that Gemini Killer caused when he took Karras's body.
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.
The silver medal in that department has to be the "The power of Christ compels you!!" scene.
Some feel that the possessed Regan's voice has aged badly, since nowadays the voice of a Creepy Child often seems creepier than something inhuman. A good number of the comments to this video are saying that they find Linda Blair's real voice creepier than the dub.
The pea soup vomit.
Nausea Fuel: Most people were physically ill and fled theaters during the medical scenes, especially the spinal tap.