My comments policy
A short answer would be to point to what most comment threads on the internet look like, in cases where moderation is not used. Discussions veer off topic; insults fly; people call each other names, accuse each other of things, and get into meta-arguments about what exactly each party meant by what he said. People post comments incorporating long, turgid excerpts from external articles. Sometimes there are statements a reasonable person could consider defamatory. I really don't care to let my own site turn into that, or even take any risk of it doing so.
This is my space; other have, or can easily create, their own. I therefore reserve the right to reject any comment on the grounds of irrelevance, insulting language, threatening language (threats which I deem serious will of course be reported to the relevant authorities), use of obscenity, or, to be blunt, anything else I happen to strongly dislike.
This is not a freedom-of-speech issue. I neither have nor want any power to prevent you from saying anything you choose. But I am not obligated to provide you with a forum in which to say it. Blogs are easy to create. On your own blog, you can say whatever you like, unmoderated by me or anyone else. On this one -- well, as the saying goes, a man's home is his castle.
Update 1 (11 February 2007): As of today, having had this blog for almost six months, I've rejected a total of four comments -- three for being pure insults with zero information content, and one for irrelevance (a rambling screed of Koran/ hadith exegesis, of staggering length, submitted as a comment on my Iran airstrike posting). Four comments in six months isn't many. So I'm satisfied that my comments policy isn't excessively restrictive.
Update 2 (2 March 2009): Since this has come up a few times, I might as well make it explicit -- I won't put up with comments that attempt to derail my site into "debates" over delusional nonsense. It wastes time and can discourage other commenters who might have something worthwhile to say. You can't talk intelligently about the good or bad ideas of politicians while being interrupted by someone who insists that they are all part of a secret global Zionist conspiracy. You can't intelligently consider aspects of primate evolution while being interrupted by someone who insists that everything was specially created six thousand years ago. You can't coherently discuss what to do about anthropogenic global warming while being interrupted by someone who insists that anthropogenic global warming is a myth. There are plenty of places on the internet where reality-denial is accepted, even welcomed. This isn't one of them.
Update 3: Lately a new form of trolling has become rampant on many blogs -- change-the-subject trolling.