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.
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.
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.
Nausea Fuel: Most people were physically ill and fled theaters during the medical scenes, especially the spinal tap.