I have a really hard time articulating why it isn't as good, and I think it's just that once the initial shot of lunatic originality behind the idea of blue collar exterminators chasing ghosts wears off, it becomes more of a typical fantasy/comedy movie. It does have a few really great moments, though -- I had a buddy in law school who would endlessly quote Louis Tully's opening statement at their trial. "One time I turned into a dog, and they helped me. Thank you."
It is definitely superior to the remake, but then again I would rather watch autopsy videos than Fembusters.
I should hate Ghostbusters 2 because it does so many things I hate about some sequels. There are some sequels that basically just make the same exact movie as the first one, but they change one variable (IE the setting) to make it seem different when it really isn't. In the case of Ghostbusters 2, it is almost a beat for beat rehash of the first film...but it has a baby in it! So that makes it different! Think back to Home Alone 2 (Same exact movie as the first, but in New York!) or good God almighty, what about Hangover 2? That one felt like they just played mad libs with the script of the first and then just switched out all the variables and presented as a different movie, even though it plays out exactly the same.
Here, the Ghostbusters have to put the business back together, we see them go through another successful montage of capturing ghosts after a rousing first successful capture, the villain targets Dana Barrett which pulls the Ghostbusters onto the case, Peter tries to win over Dana (again), and there is a giant in the climax (in this case, The Statue of Liberty, used for the side of good this time). Its basically the same movie.
And this always bugged me, but I hated that they romantically paired up Jeanine and Louis when in the first film and the cartoon show, Egon and Jeanine were the budding couple. Yeah, the fucked with my ship and I got miffed. Sue me. There are other elements that don't quite work as well as the first either. Louis' side plot of trying to become a Ghostbuster ultimately doesn't amount to much as he does virtually nothing. And Vigo, while intimidating, never quite feels like the apocalyptic threat that Gozer was.
However, there are elements to it that I appreciated. Peter McNichol is a hoot in the film, and signs of the great chemistry the cast had in the first film are definitely there from time to time. I think the big problem here is that the movie tried to play it safe by making a movie too similar to the first, but maybe softened up a bit. Tub scene aside, 2 was not as scary as the first either which gave this film less of a bite.
Honestly, there is an episode of The Real Ghostbusters cartoon show that would have actually been a better sequel if it was made into a movie. In "Citizen Ghost" the Ghostbusters' suits that were covered in Gozer's marshmallow mess in the climax of the first film come to life as evil spiritual forms of the Ghostbusters and start running amuck throughout the city with their own proton packs and everything. If they had made this into a movie, you would have seen the Ghostbusters confront evil versions of themselves, and even become allies with a ghost (This episode was the one that detailed how Slimer joined the gang). To expand on for a movie, they could have had each Ghostbuster have to deal with some personal issue that the evil version becomes an outward representation of and they'd have to conquer those fears and inadequacies. Lets say Peter has trouble committing to Dana because he likes being a free skirt-chasing bachelor, and the evil version of him represents his greedy and womanizing side, and Egon has fears of intimacy as he can't admit his feelings for Jeanine and buries himself in his work (see I'm still trying to make that ship happen), so the evil Egon is a cold, calculating, intelligent, and heartless monster, and so on and so on. I don't know, I'm just throwing ideas out there.