Creator Cameo: One of the show's writers, Wendy Molyneux, voices Jen the babysitter.
Cross-Dressing Voices: Linda's and Tina's voices are provided by John Roberts and Dan Mintz, respectively.
For that matter, most of the one-shot/background female characters seem to be played by men, too.
And then you have the Pesto twins being voiced by Sarah Silverman (Ollie) and her sister, Laura (Andy).
Hey, It's That Voice!: 95% of the main cast and the guest voices are people known from the sketch and stand-up comedy world. The other 5% are from other TV shows, particularly those that are either considered modern cult classics or earned the title of being a cult classic because the show/movie was either screwed over or caters to a nerdy, pseudo-mainstream sensibility.
Bill Hader (Officer Slater, Stefon Zolesky-Meyersnote the Club Kid who became a city correspondent for Weekend Update. When he was introduced, it was implied his last name was Zolesky, as that was also the last name of his brother Dave, played by Ben Affleck. The "Meyers" part came from Hader's final episode as a cast member when Stefon ran off to marry Anderson Cooper, and Seth Meyers ran to rescue Stefon from getting married, and Flint) is the voice of Mickey the bank robber (and of all the weird voices Bill Hader has used, Mickey the bank robber is one of the few where it's closest to his normal voice).
Tammy the bad girl is Jenny Slate, a one-time Saturday Night Live cast member who got in trouble for accidentally saying "fuckin'" during "Biker Chick Chat," her very first sketch on the season 35 premiere hosted by Megan Fox. Slate is also the voice behind "Marcel the Shell With Shoes On", was on an episode of Raising Hope as a social worker, and was Stella on the short-lived noir parody Bored to Death.
Louise is voiced by the dub actress who plays Vanellope. Meanwhile, Vanellope's English voice actress (Sarah Silverman) is one half of the Pesto twins.
What Could Have Been: In the original pitch of the show, Bob and his family were cannibals who served ground-up human meat to unwitting customers, while trying to keep the authorities off their trail. Naturally, something that disgusting got rejected as a series premise for network TV (it'd be a different story if it were on cable — either basic or pay-premium), but it didn't stop the first episode from alluding to it.
Tina was also supposed to be a boy named Daniel, which explains why Dan Mintz (a boy) is voicing a girl.
John Roberts originally auditioned to be a cast member for Saturday Night Live during the 2008-2009 season (when Amy Poehler left to have her baby and do other projects, so Lorne hired Bobby Moynihan, Abby Elliott, and Michaela Watkins). Had he been chosen, not only would John Roberts be the second male homosexual cast member (and the first one hired since 1985), but he wouldn't have been the voice for Linda.