"I am unprepared for people who don't understand jokes" -- Anton Troynikov I don't know who he is but I really like this quote. That's consistently my biggest surprise on the internet. The #1 question I get for my posts is "Is that sarcasm?" I never answer.
Sarcasm and humor are a big deal. They are active attempts to identify resonance with a crowd and common ground, and can be used to assess if there is a strong cultural basis in a community. In the internet it's become very hard to have cohesive communities, so sarcasm has to be labeled (perhaps hoping that it will help people learn to identify it without labels in the future - in vain, i think).
Sarcasm can be culturally dependent. As a child and high schooler, I thought that my parents didn't understand sarcasm. I remember when I realized that my immigrant parents just had a different form of it than I did in grade school and high school.