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 (vide the Autoadmit fiasco). 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 or Volokh Conspiracy 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. 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. (This blog is certainly not as highly trafficked as some, but it has, shall we say, an unusually 'high quality' audience, at least for anyone wanting to reach Deans and law faculty.) 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. 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, especially since--notwithstanding the reprehensible Section 230 of the Communications Decency Act which insulates me from liabilty for any of the defamation or tortious material that might show up in the comments of my blog--I'd rather not have a site bearing my name be the repository for the kind of garbage that is typical on the blogs that do not moderate comments.