Bile Fascination: The books about going potty. They are meant to be used to potty train toddlers. But they became popular with teens and adults because of the potential toilet humor material - to wit, one book has a sound module that plays the sound of a toilet flushing. Another comes with a sticker of a medal proclaiming "I'm a Toileteer!". Both has potential for abuse.
Ojo also counts to a smaller extent, due to being the only girl of the group.
Fridge Logic: How is it that a group of kids that all or most apparently have no parents never feel a need to talk about that?
Germans Love David Hasselhoff: This series was huge in the United Kingdom. It consistently topped preschooler television polls when it aired and continued receiving DVD releases long after the show ended. It even got a kiddie ride made by Fun2Learn. In the US, Disney wouldn't even reissue episodes onto DVD.
Ho Yay: Take Tutter's quote "Thank you, Beaaaar!" out of context and you'll likely get this.
He Really Can Act: Lynne Thigpen as Luna can really sing. It's especially poignant if you knew her best as The Chief, who never sang.
Squick: From the "Brush Brush Bree" song with the characters' brushing their teeth, Pip's "Look, Pop, I found a clam!" Even the other characters other than Pop are grossed out.
Bear, Tutter, Treelo and Ojo: Ew!
Several moments from "When You've Got to Go", including Ojo having a Potty Failure, having a club called the "Toileteers", mentioning animals pooping, and several moments from the Potty Chair song.
Tastes Like Diabetes: The show occasionally veer towards this with things like the "Dear Grandma" song. It never quite seems to get there, though, possibly because it's just too sincere.
They Wasted a Perfectly Good Character: There were a number of regular guests that they probably could have developed more or even become regulars, but on the whole they generally stuck with focusing for the most part on the main cast.