The New York Ripper is an acquired taste. In it, a serial killer who's adopted a Donald Duck voice to taunt the police is murdering beautiful young women in The Big Apple. That's it. That's the entire setup.
The immediately apparent aspect of movies like The New York Ripper is the surrealism, which is likely the byproduct of an Italian film crew making an "American" movie. A foreign actor whose accent has been dubbed over should, in theory, be undetectable, but that's never the case. And although Fulci and company drag their handheld cameras to the darkest corners of NYC, it doesn't quite look like the city we've seen in countless movies. Some of the indoor locations were probably filmed in Italy, but even the stuff that's undeniably New York feels... off.
"Off" is an effective feeling in horror movies.
Too many people maintain that a movie is always more effective when it only implies the carnage. If that were strictly true, why do the crusaders only rally against the ones that dare to show it? Better yet, why can't we just appreciate both types of movies?