A serious post (I know, it's hard to tell at this point, isn't it?):
Why does HN need the karma points system at all? Downvoting is hardly used on other social networking sites any more, which should tell you something. Getting one pedantipoint has to be a motivation for "I have stuff to say about JIT." posts.
Blocking & muting people is quite effective on Twitter, for example. The dream of a civil discourse with people disagreeing politely was obsolete before the Web was invented. There are a lot of accounts which someone here would rather never see, so why not just let those people erase them, just for their own TL?
Automatically downvoting anything you don't agree with is, basically, "Lord of the Flies" behavior. It leads to brigading.