1. Sarcasm is often difficult to discern without tone of voice.

2. Humor for its own sake is less valuable than humor that makes a point. I wish I could find it, but I read an interview with the New Yorker's cartoon editor who said something similar.

3. It is easy to tear down with sarcasm but very hard to build with it. There is too much negativity in anonymous / pseudonymous forums in general.