Why is it that any time any discussion about D&D comes up which could even remotely somehow be construed as some kind of criticism of D&D as-is, a select few of the forum regulars swoop down with such venomous, vicious spiteful attacks to shut it down?

I've been on every kind of forum, I currently spend time on historical, technical, and even political forums. On some of these, peoples entire professional reputations can be at stake based on the credibility of say an article they wrote which is under discussion. And yet, while you can always get the odd forum crazy and find a certain amount of intractibility in discussions, no other forum I know of is anywhere near as vicious as an RPG forum. I can honestly say that the RPG forums, especially any D&D forum, are both the most vicious and the most utterly intractable and cynical in terms of peoples positions.

Anyone who knows these forums knows that unless you are ready for a big fight, to avoid a WIDE swath of subjects, or else face the near certainty that your thread will be hijacked (as mine recently was) into a completely off-topic diatribe of insults and vicious demands to "love it or leave it" until the temperature of the "debate" inevitably gets hot enough that the moderators shut the thread down, which is exactly what the attackers want from the start.

The net result is a de-facto censorship. This is in many ways a great forum, but there cannot be any rational discussion about anything real about D&D in here in terms of improving it in any way or addressing anything but techincal or balance problems. Anything else will instantly turn into an all-or-nothing debate about D&D.

Why does a role playing game bring out such intense feelings of hostility? What is it about D&D that makes people to devote hours to willing to insult and ridicule complete strangers? What is the ultimate effect of this self-censorship and passive aggression on the game itself?