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.
Executive Meddling: Despite its runaway success, ABC and the focus group Q5 decided they needed to "fix" the show. And not just once. Janine was changed to be more feminine and meek, and received round glasses because her pointy ones could scare children. More changes were forced upon the writing staff starting with Season 3. Recurring child characters were added. More focus was placed on Slimer, who spun off into his own (doomed) cartoon in Season 4. The whole team ended up taking on fixed roles: Egon was "The Brain", Ray was "The Hands", Peter was "The Mouth" and Winston was "The Driver". This was enough to make J. Michael Straczynski, the head writer at the time, 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 2 for a Janine/Louis relationship.
Q5 had also wanted Straczynski and the other writers to ditch Ray, as they felt he was a pointless character. However, this was one change that was quickly and successfully vetoed.
According to The Other Wiki, initially they wanted Maurice not to do a Harold Ramis impression, but as an impressionist, Maurice felt he couldn't properly play Egon without following Ramis' example (he won the part anyway with it). He also tried a "Woody Allen" approach, but it didn't fit Egon's personality.
The Slimer! shorts featured Cree Summer as Chilly Cooper, an ice cream truck driver who befriends Slimer.
Crowley from "The Halloween Door" was voiced by Michael Rye, better known as Duke Igthorn.
In "Ghostworld" Mrs. Spengler was voiced by Rose Marie, better known as Sally Rogers. An earlier appearance near the end of an episode had Maurice LaMarche voice her for the last few seconds when she sent the guys out on a call.
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.
Ernie Hudson (Winston in the movies) auditioned for his own character.
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").
An early six minute demo reel featured an entirely different character design for Peter Venkman (one that came closer to the more blue collar looks of live action Venkman Bill Murray than the chiseled pretty boy design they ultimately went with), the Ghostbusters wearing their beige movie jumpsuits instead of the multi-color coded jumpsuits they wore in the finished series, and Slimer still appeared to be an antagonist rather than a team pet and mascot.