I sort of hate to continue this thread, but that is definitely my rule for email. I decided a few years ago that, if it came down to it, I'd rather come across as an over-exuberant (can't think of a polite word for 'idiot') than come across as an asshole. Most of the time, at least.

The upside is that if you add an exclamation point, you can get away with much shorter messages!

A smiley works as well :)

No no no no. Take people seriously on the Internet, at face value, and eventually people will realize that sarcasm doesn't work and will stop using it.