Anonymity + Audience = http://www.penny-arcade.com/comic/2004/03/19
1) Its curated. There is someone to separate out the drivel. And because its curated and people know that, they're less likely to send in complete crap.
2) It relies on physical media. It takes some level of effort to make a card and send it in not to mention a small but non-zero cost. It also means that its not instantaneous like most things online.
3) There is no real feedback. There is no direct comment system, so theres no way to feed the trolls.
One of my closest friends is dyslexic, several others are in separate countries, and we've all got various tics and tells we might not even be aware of.
On the other hand, if this is with 'friends' and not completely random audience things may manifest quite differently.
