[in the Comments section] you will find the instruction
Be relevant. As bloggers, we write about whatever we want to. As a commenter, you should comment on the contents of the post you're commenting on. If you want to write about something else, do it on your own blog.
Commenters have violated this injunction again and again (for reasons I think I understand). The comments policy goes on to say
Comments that violate these guidelines will be deleted. Repeat offenders may be banned.
but in fact we've been extraordinarily tolerant of errant comments, even allowing comments that explicitly introduce topics that have nothing to do with the topic of the original posting. These are the most flagrant violations, but there are more subtle ones.
The flagrant violations begin with something like "This is off topic, but…", sometimes adding that the writer didn't know where to post this observation. (The short answer is: NOT HERE.)...
What goes wrong? Our comments policy was designed to keep focus on a particular topic, but Language Log is treated like any blog that allows comments, unmoderated mailing list, or unmoderated newsgroup: everything is open space. So discussion wanders all over the place. That's the way the net works, and there's probably not anything we can do about it. Asking commenters to be brief, relevant, informed, and polite (as our comments policy does) is asking them to behave in what is now an unnatural way on the net.
(Yes, some commenters are scrupulous about adhering to our comments policy; I'm not saying that all commenters behave badly, but a surprising number do.)
(I was invited to write a guest post for LL a couple of months ago, which gave me the chance to stop overstocking the drafts folder on pp and start building up a virtual drafts folder for Language Log instead - bad, bad, very bad. And one includes quotations from L. D. Reynolds' introduction to his OCT edition of Sallust, something readers are sure to enjoy and unlikely to see anywhere else, so something must be done (but not, needless to say, today).)