Anonymity

The Internet gives more people a voice than ever before. Unfortunately some of them have a seemingly endless reservoir of vitriol and derision that they've tapped with a fire hose and are eager to spray all over anything they lay eyes on. Reproving responses only seem to make it worse, while ignoring them seems to deprive them of whatever reward they glean from this behavior.
Another interesting thing about this electronic forum is that it's much harder to get by on your looks. People are more likely to evaluate you based on your intelligence, strength of personality, and communicative skills. How cool is that?