Strong and interesting communities have in-jokes. These shared references are exclusionary – you have to get the reference and know the backstory to enjoy the joke.

If you don't, you're left mystified, but that deepens the lines around the community – the sense of who it's for, and who it's not.

Superbly put!

Often ignored aspect of interaction in a community. Those who are in thinks its just natural, those who are out feel this is the way to show them they are out.