A)There's tons of conversation going on here in this topic and B)Burn anyone at the stake who wants to make a post that is less than serious (I think this is my one and only time doing that on HN).
If I understand correctly they are per thread and disabled by default.
Paul Graham wrote in one of his essays about Hacker News:
The most dangerous form of stupid comment is not the long but mistaken argument, but the dumb joke. … Whatever the cause, stupid comments tend to be short. And since it's hard to write a short comment that's distinguished for the amount of information it conveys, people try to distinguish them instead by being funny.
… Bad comments are like kudzu: they take over rapidly. Comments have much more effect on new comments than submissions have on new submissions. If someone submits a lame article, the other submissions don't all become lame. But if someone posts a stupid comment on a thread, that sets the tone for the region around it. People reply to dumb jokes with dumb jokes.
(What I've Learned From Hacker News: http://www.paulgraham.com/hackernews.html)