There's so much more to making a game than designing monsters and damage values. In fact, so much of what makes a modern videogame "click" are systems that the average gamer will never even think about. Which is why this thread by globe-hopping game designer Jennifer Scheurle (who you can follow @Gaohmee) is so enlightening. She asked for devs to tell her about the mechanics they are most proud of that were explicitly HIDDEN from their intended audience, and how it enhanced their experience.
There's thousands of responses in the thread, but here's 12 that made us go "OF COURSE THAT'S HOW THAT WORKED".