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," ten 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 ten 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 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, "Back in the Saddle," has one scene in which the WTC 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 ten years before the now-infamous PRISM surveillance programs were launched. (Of course, the fear of such surveillance goes back as far as FidoNet...)
Harsher in Hindsight: The premise of having Egon as the only active member of the Ghostbusters can seem a bit darker since Harold Ramis (Egon's original actor) was the first of the original Ghostbusters to pass on in 2014.
Hilarious in Hindsight: In the episode "Home is Where the Horror Is", one of the children asks Roland if he is Batman. The voice actors of most of the Extreme Ghostbusters would later have roles on The Batman (Garrett's actor Jason Marsden playing Firefly, Kylie's actress Tara Strong playing Vicki Vale in the direct-to-video movie The Batman vs. Dracula, and Eduardo's actor Rino Romano playing the Dark Knight himself), with Roland's actor Alphonso Robero being the only one not to have a role on the series, making the boy's question more hilarious these days.
In particular, there exists a version of the outro sequence without the theme song, and only has some ambience while the viewer is presented with several disturbingly-detailed sketches of the series' supernatural beasties while some legitimately creepy "ghost sounds" are played back. Without the amazing theme song to groove to, the "ghost sounds" make the presentation of the ghost artwork extra spooky. Several viewers, including some commenters on YouTube, have admitted to being spooked by this "no-BGM" outro for years.
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. Most memorably, he tried to pull a Heroic Sacrifice in one episode.
Janine, likewise, is a "best of both worlds" version of her fluctuating character design and personality from the original series. She looks like her Season 4/5 version (which was considered permanent after her Deal with the Devil), but acts like an Older and Wiser Season 1 version.
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 (though the title could refer to the fact that the newer Ghostbusters are more badass and face more frightening ghosts than the original ones, rather than some executive's feckless attempt that making them "cool"). The racially/sexually diverse cast was another attempt to keep the show relevant to the 90's (though it doesn't make the show that dated, especially in this day and age when people are calling for more diversity in their entertainment).
What Do You Mean, It's for Kids?: The show had an even bigger degree of Nightmare Fuel than its predecessor, and in earlier episodes had characters implied to actually die (albeit offscreen). This is probably why they felt forced to establish in later episodes that trapping a ghost caused a Reset Button.