It's hard for me to see Ghostbusters as horror for the same reason it's hard for me to see Buffy the Vampire Slayer as horror. Horror implies a sense of helplessness and, well, terror in the face of the monstrous or supernatural, but the Ghostbusters and Buffy's Scooby Gang don't run or hide from the monsters, they hunt them down and overcome them. Okay, there are a lot of horror movies where the heroes (or in the '50s, the police or military called in by the heroes) destroy the monsters in the climax, but it's usually after a fair amount of death and despair and fleeing in terror. I see Buffy as more of a superhero series, and I guess you could make the case that the Ghostbusters qualify too, in a way.
In particular, what's always been intriguing to me about Ghostbusters is how it treats the paranormal as simply a scientific problem to solve -- and something as mundane as an exterminator business.