As longtime readers know, I open comments selectively when I think reader input is likely to be especially informative and when I have time to moderate comments. This practice has worked well, in terms of the quality of the comment sections, but it is time-consuming and it also means that relatively few threads have commenting opportunities. But it seems to me that more laissez-faire policies have significant disadvantages.
First, there are likely to be far more anonymous comments, and anonymity generally encourages irresponsible behavior. Second, there would be a lot more spam--a lot of older threads with open comments get spam fairly regularly, but that never sees the light of day under the current system. Third, the quality of threads is likely to be much more uneven--take a look at Crooked Timber threads to get an idea of what tends to happen. There are some folks who comment rather excessively on any blog where the opportunity presents itself, and what they have in common is rarely skill and insight. (Search "John Emerson" on Crooked Timber for an example of the problem.) The comment sections of highly-trafficked blogs are very attractive for those who want attention, and especially if their professional competence does not permit them to get such recognition from established fora outside the blogosphere. And some nuisance commenters are just literally nuts. (Pharyngula maintains a whole list of permanently blocked commenters, including some denizens of Cyberspace whom he deems, not implausibly, to be mentally ill--a phenomenon, needless to say, with which I have some familiarity.)
The question, of course, is how likely it is an "open" comments policy here would devolve in these ways. When there was more scathing political commentary on the blog, that risk was certainly greater, but these days the quality of submitted comments are pretty good, and I approve at least 90%. On the other hand, the amount of garbage would surely increase if there were not the specter of comment moderation, so for the foreseeable future I intend to maintain the comments status quo. If you have thoughts on the issue, feel free to e-mail me. Thanks.
UPDATE: Sentiment so far is strongly in favor of continuing moderation. One student gave apt expression to the case for moderation:
Per your recent post, I think it would be better if you kept moderating comments as you have. The vast majority of discussion on the internet is worthless, your blog being a rare exception. If comments are not moderated it is safe to assume not only that there will be more junk comments, but that this decrease in average quality will cause a decrease in the number of worthwhile comments as well. Many of the most informative and best-considered comments in current discussions come from well-respected, and no doubt very busy, philosophers, whom I imagine would have little desire to spend their scant free time engaging with belligerent internet trolls.
Pretty much my sentiments exactly.