First, and most importantly, there are absolutely no rules about what can be written (or drawn) on a card.
But what constitutes a “good” Otherfoot card? This is hard to pin down, and will vary a little from one group to another. The best general advice is this: Don’t try too hard to be funny. If you’re new to Otherfoot, there’s often a temptation to come up with cards that are self-contained jokes. However, most of the humor in Otherfoot is contextual, based on unexpected combinations of cards. Don’t try and tell an entire story on each card. “Half-baked” ideas usually work better.
Take, for example, a card saying “Richard Nixon was a lizard person”: OK, that’s pretty funny, but where do you go from there? You’ve already told a complete story! What if, instead, you had written “The one thing that could have saved Nixon”? The possibilities are more open-ended here. Maybe it’s “karaoke competition,” “grandma’s pesto,” “fedora guy,” “intersectional feminism,” or “just a big bowl of Frosted Flakes.”
So what makes for good subject matter? Every group is different. What one group finds uproariously funny might fall flat with another. This is one of the best things about Otherfoot! Play it with a different group of people, and it’s a totally different game. Each group of players gets to work from their own unique blend of humor.
Need some more examples?
Paul Duffell and Andrew Macfarlane penned this 2005 writeup on Otherfoot, which contains a few classic card examples. Also, @otherfootbot randomly tweets Otherfoot pairs from a list every 3 hours (yeah, I know, prefab list, kind of defeats the purpose, but still).
But seriously, don’t feel constrained by these particular examples. Get creative! Make the game your own!