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.
The ectoplasmic being known only as "Piper," from "The Pied Piper of Manhattan," initially seems a savior to New York City when introduced, as his pipe has the ability to repel ghosts even the Ghostbusters are powerless against. Piper soon reveals his true colors, however: he is the one responsible for the sudden plague of ghosts, and when the Mayor attempts to pay him according to contract, Piper reveals he wants more. After his requests for a statue of himself down 5th Avenue and an office in city hall are denied, Piper uses his flute to hypnotize the children of the city and tries to force them to drown themselves, gloating the mayor will "rue the day he refused to pay the Piper." When the Ghostbusters intervene, Piper has no compunction using the kids as human shields in the ensuing fight.
The Grundel from The Real Ghostbusters returns in "Grundelesque," 10 years after he was captured by the original Ghostbusters. It is revealed that he has a connection to one of the new Ghostbusters, Kylie. The Grundel tried to corrupt Kylie as a child, but she was too strong willed, so he corrupted her friend Jack instead. Deciding that Kylie's influence will mean that Jack will not be a keeper, the Grundel places Jack in a cocoon for 10 years, so that he will become another Grundel, thus managing to steal ten years of Jack's life. When Kylie confronts the original Grundel about this new Grundel, the Grundel taunts Kylie about their past history and the fate of her former childhood friend. The Grundel later escapes from the containment unit and tries to corrupt Roland's brother Casey. When Kylie manages to confront the Grundel, Kylie says she is not afraid of him, because his powers only work on children. The Grundel knows this, but thinks it would be fun to murder Kylie anyway.
"Funny Aneurysm" Moment: Of the "tasteless after the September 11th attacks" variety: The second part of the series finale episode, "Back in the Saddle," has one scene in which the World Trade Center towers are being devoured by the blob-like ghost that's invading New York City.
In the episode "Grease", Kylie offhandedly warns the others to not spend so much time on the Internet, because the government could likely see and track everything they do. She's even reading a huge book about conspiracies in the scene. It doesn't seem like much, until you remember this was a cartoon that aired in 1997, a whole 10 years before the now-infamous PRISM surveillance programs were launched. (Of course, the fear of such surveillance goes back as far as Fido Net ...)
Harsher in Hindsight/Irony: The premise of having Egon as the only active member of the Ghostbusters can seem a bit darker now that Harold Ramis (Egon's original actor) passed away, leaving only three of the original Ghostbusters alive.
Additionally, his characterization is more in line with the early days of the prior series. Rather than act like a Cousin Oliver, he behaves the way that made fans like him in the first place: a Big Eater that speaks incoherently and tries to do his best to help his friends.
We're Still Relevant, Dammit: Despite putting teenaged charactersnote Though, considering that they all go to college, it's safe to say that they're at least between 18 and 22 — the typical age bracket for college students front and center, the series generally avoided 'totally radical' territory. Except for the title. The diverse cast was another attempt to keep the show relevant to the 90's.
Fridge Brilliance: Ah, but, the Ghostbusters themselves are not "extreme." The ghosts are.