California Doubling: The 1975 series episode "The Dummy's Revenge" has Spenser, Kong and Tracy entering a building lobby to check someone's contact information. That lobby was actually the foyer of the Filmation building!
Creator Cameo: Filmation founder Lou Scheimer as Zero in the live-action series and several voices in the animated series. Tracy, Fib Face and Ansabone are but three examples.
Marth Debuted in "Smash Bros.": At least outside the U.S., since the animated sequel was broadcast overseas, while the original live-action TV series prequel was never exported anywhere.
Reality Subtext: The episode "Statue of Liberty" revolves around the newly-restored Statue of Liberty, which Americans had celebrated in real life that past July. (The episode aired in September 1986, just two months after "Liberty Weekend.")
There's a sequence in the episode "Statue of Liberty" in which The Ghostbusters go up a reeeeeally long flight of stairs in pursuit of Mysteria. Mysteria catches up to Eddie and surrounds him with mist. Eddie falls, and Tracy uses the Ghost Gummer to catch him in the nick of time. The sequence was reused in a later episode, replacing Mysteria with a generic orange ghost.
The Other Darrin: In the 1975 series Jake Kong Sr. was played by Forrest Tucker and Eddie Spencer Sr. was Larry Storch but in the 1986 series and probably due to the passing of Forrest Tucker, Jake Sr. is voiced by Pat Fraley and Eddie Sr. is voiced by Peter Cullen.
In the original "series bible" (the document which lays out the vision of the series), the moronic Airhead was described as working for the Ghostbusters while serving Prime Evil—in other words, a mole. This gave his incompetence a very interesting angle, but in the finished series, he was just a moron.
In her original design, Futura was an African-American with long, light-brown hair, and Jessica Wray originally had blonde hair.
Prime Evil was originally designed to resemble Mumm-Ra, while Scared Stiff was originally conceived as more of a "walking fridge" robot and Mysteria originally had a human appearance and a long red dress.
Tex-Hex from Bravestarr was originally meant to be a Ghostbusters villain. Lou Scheimer took one look at his character-sketch and decided that he should be a Big Bad in his own right.
The original 1975 series did very well; it came in second for ratings, right behind another Filmation live-action show, The Shazam/Isis Hour. A second season could have been done but Filmation cancelled it instead to concentrate on the aforementioned show.
Most likely the animators were told to time the intro animation to the Ray Parker Jr. music just in case a last minute agreement with Columbia Pictures (or case handed down in court) allowed them to use it.