I've written before about the value of filtering out abusive or otherwise unconstructive comments for the sake of promoting reasoned discussion. It's an easy decision to delete comments that don't contribute to the conversation at all. I'm less sure what to do about relevant but low-quality comments. At present I tend to just leave them there but ignore them. (Of course, sometimes I fail to respond to good comments for other reasons, e.g. lack of time.)
But this liberal policy has some unfortunate consequences. The inclusion of low-quality comments may bring down the quality of discussion overall. As Tyler Cowen observes, "the best comments come in the first fifteen or so, after which quality declines precipitously and often exponentially." Short comments are no problem, I think, but long rambling ones are a disincentive for later readers. (One may feel obligated to read the earlier comments before responding, or even feel less comfortable responding to the original post after the conversation has apparently "moved on" to the rambler's choice of tangent.)
The ideal solution would be to have graded levels of comment prominence. I like blog templates which highlight the blogger's own comments relative to the norm. It would also be helpful to be able to make some particular comments less prominent than the norm. The text might be 'greyed out', for example, paled or even completely hidden by default unless readers select to 'click here to display this comment', or some such. Most readers could thus easily ignore the comment, reducing the risk that it will derail the thread, without resorting to heavy-handed deletion (which risks provoking [misguided] complaints of "censorship").
[I've posted this feature suggestion to the Blogger help group, and anyone who likes it is also encouraged to tell Blogger, since they will be more likely to implement a feature for which there is more demand.]
In the meantime, I'm tempted by the following policy: if a comment is both lengthy (i.e. more than a couple of sentences) and unproductive (by my lights), I may delete it and invite the commenter to instead repost the comment on their own blog -- they are then welcome to post the link in my comment thread, since that is far less obtrusive and distracting. (I'm happy to email the person their deleted comment, of course, so they needn't worry about having to rewrite it from scratch.)
Feedback welcome: Is this a good policy -- would it serve to increase the quality of discussions on this blog (which I think is generally quite high to begin with)? Does it seem reasonable? All things considered, do you think I should implement it?