Acting for Two: Of the main characters, Frank Welker voiced both Ray and Slimer. Welker would also voice a number of antagonists, such as the Bogeyman, Sandman and Samhain. Maurice LaMarche also voiced a number of roles, too. Lorenzo Music also voiced Peter's dad, with Dave Coulier taking over later. And since Laura Summer (who originally played Janine) is the only voice actress credited, she may well have played every female role in the series until she was replaced by Kath Soucie.
In "A Fright at the Opera", before the Diva selects Peter as her bodyguard, she states "I hate Mondays". Lorenzo Music's (Peter's voice actor) most famous role, Garfield, also has a distaste for that particular day.
Creator's Pest: The Junior Ghostbusters were despised by the writers. J. Michael Straczynski in particular has gone on record stating that he'd only use them in an episode if he were allowed to have them run over by a truck.
The Danza: In the Slimer! short "The Not-So-Great Outdoors", Rob Paulsen plays a rabbit named Robby.
Executive Meddling: Despite its runaway success, ABC decided they needed to "fix" the show and hired a consulting firm named Q5 to take a look at it. According to J. Michael Straczynski, the head writer at the time, Q5 didn't do any audience research or focus testing but demanded changes based on what they themselves believed would made a cartoon successful. Janine was changed to be more feminine and meek, and received round glasses because it was claimed her pointy ones could scare children. More changes were forced upon the writing staff starting with Season 3. The Junior Ghostbusters, a group of recurring child characters, were added. More focus was placed on Slimer, who spun off into his own (doomed) cartoon in Season 4. The consultants also wanted fixed roles for the four main characters with Egon as "The Brain", Ray as "The Hands", Peter as "The Mouth" and Winston as "The Driver" (the racist implications of which Straczynski and the writing team rightly called Q5 out for). This was enough to make Straczynski leave his position after Season 3 (though he wrote some later scripts for the show as a free-lance writer).
The only reason the Egon/Janine relationship was never allowed to go anywhere? Harold Ramis hated the idea, and since the actors had creative control over their characters, it was vetoed. This is why it was totally dropped in Ghostbusters II for a Janine/Louis relationship.
Ghosts were also made to be less human and more monster-like due to the network thinking that kids couldn't handle death. Demons were also cut so as to not expose them to demonic themes and to avoid offending anybody's religious sensibilities.
The recasting of Peter's voice actor from Lorenzo Music to Dave Coulier in Season 3 was that Bill Murray complained to the studio that the character sounded too much like Garfield. note Ironically, Bill would go on to voice Garfield in the live action movies, largely out of penance for this act since he did like Lorenzo.
Q5 had also wanted Straczynski and the other writers to ditch Ray, as they felt he was a pointless character. Ray had always been the warm, friendly one, but by this time, the other cast had been rendered so safe and bland, they decided he was redundant. However, this was one change that was quickly and successfully vetoed.
One of the most jarring changes was Peter could no longer be mean to Slimer.
Missing Episode: Every episode was included in the now out-of-print Time-Life DVD set. However, when the rights reverted back to Sony and they released 10 single-disc volumes to promote the 2016 film, only 111 out of the 173 episodes (counting the Slimer episodes) were included. Five of the remaining episodes did become available only for digital download.
The Other Darrin: After the syndicated episodes, Dave Coulier replaced Lorenzo Music as Peter and Kath Soucie replaced Laura Summer as Janine. A season later, Arsenio Hall left for his talk show and was replaced by Buster Jones.
In the second season, whenever the Stay Puft Marshmallow Man appeared, Frank Welker handled any vocal effects. In Season 3's "Sticky Business," John Stocker took over the role. Later in Season 5's "Partners in Slime," Welker again voiced the character for a brief cameo.
Out of Order: Comparing the syndicated episode premiere dates and the DVD ordering, one can see a multitude of differences. Due to its stand-alone nature, this would've hardly been noticeable for the series, except for two major examples:
"Cold Cash and Hot Water" premiered before both "Venkman's Ghost Repellers" and "The Spirit of Aunt Lois" - despite those two formally introducing Peter's dad and Doctor Basingame, respectively. ("Cold Cash and Hot Water" even references Basingame's bogus seance at Aunt Lois' house.)
"Slimer, Is That You?" and "Transylvanian Homesick Blues" features a weird example. The former was produced as part of the syndicated package and the latter for ABC. However, the two were somehow flipped, with "Slimer, Is That You?" premiering September 26, 1987 on ABC and "Transylvanian Homesick Blues" December 11, 1987 in syndication (the last premiere episode in the syndication run, no less.) It's largely noticeable due to the different voices for Peter and Janine (though the latter maintained her original design in both episodes).
Many viewers were likely confused when they first saw the Syndicated episodes that had Peter and Janine with their original voices, and the latter with her original look (they began the same time as the 2nd ABC season), unaware that they had been in the can for a year, and some of them were released on VHS by Magic Window, the children's division of RCA/Columbia Pictures Home Video, before they were broadcast.
Show Accuracy/Toy Accuracy: Strangely, the Kenner toy line had very few ghosts who had actually appeared in the show. Exceptions to this were often lacking in accuracy: the 'Slimer' figure was named Green Ghost and looked more like his film counterpart, having been designed and marketed before the show had even come up with his name; and the original Stay Puft figure is about the same height as the 'busters themselves (the latter was eventually reissued as a plushie that was more accurately sized relative to the Ghostbuster figures- not that it would be physically possible to make a toy of the character to scale with the figures what with the character having been about 100' tall, and even today with a 2' tall Mattel foam figure or Diamond Select coin bank, it's still not quite happened). The likenesses of the Ghostbusters themselves also shares a bit with the early demo reel (as described below) which is why the Ray figure is so fat and why Winston's face is off-model.
Trendmasters would eventually make a toy of Samhain in the Extreme Ghostbusters line... albeit rather off-model to his animated look.
Mattel did release a series of 9" action figures with much improved likenesses around the year 2010, although it only saw 6 toys, but it did feature a proper Samhain, FINALLY. Art Asylum also released a line of Minimates that tied in with both the film and cartoon which featured a few ghosts in toy form for the first time.
Technology Marches On: Thanks to the corporate espionage perpetrated by Paul Smart, a super-advanced, AI-driven Robo-Buster X1 was set to put the "dinosaur" Ghostbusters out of business. At its unveiling, the robot revealed it had an incredible 20MB of on-board memory.
In light of falling ratings after the Slimer Re Tool, producers asked JMS to return as head writer for Season 6. JMS had other projects at the time, though he agreed to write a few episodes (chief among them "Janine, You've Changed").
The executives softened Janine's character to be more "motherly" but they also wanted to make her a full-fledged Ghostbuster as opposed to just a secretary who only occasionally got involved with the plot. Quite a few fans think this might have actually been a good idea, especially if it had been thought up during the show's peak.
A planned episode "Funny You Should Scream" which was about Dr. Teufel (meaning devil) ran a carnival and trapped children in his Fun House a la Pied Piper. The script for the episode was scrapped due to conflict with the animation writers and the Labor Board at the time.