Animation Age Ghetto: What got the show canceled in its initial run. Moral Guardians didn't like that a cartoon was dealing with family unfriendly subjects, even if it was explicitly not for families. It didn't help that the show was originally designed for, and for the most part aired in, the new 7:30 family slot.
"Seinfeld" Is Unfunny: It can be difficult to appreciate how risky it was just to do an animated series exclusively for adults in the 1970s after their resurgence in the 90s.
Values Resonance: Despite dating itself by taking on contemporary issues, much of the edgier content has held up (mostly because several of the issues, for better or worse, never really went away so much as took on new forms). Some of the show's handling of feminism (especially the episode where Irma gets a job at a law firm and the younger, prettier secretary is treated better than the older, plainer Irma), and population control ("Expectant Papa") are still relevant in the 21st century. It helps that the show never stooped to the kind of scatological or racist humor that many animated shows for adults rely too heavily on.