Forget about MR and its superb commentators, I am talking about the typical above-average blogs. I often have the impression that the best comments come in the first fifteen or so, after which quality declines precipitously and often exponentially. Why might that be?
1. The truly smart people only like to make smart points on "fresh" posts. For instance more people read the comments on fresh posts (but why?), so the
benefit of a quality comment is lower as the post becomes older.
2. As time passes, the chance that a warring twosome find each other, and take over the thread, increases.
3. There is a tendency to attack or respond to the stupidest or most controversial thing said, and the longer the comments thread runs for, the stupider this will get.
4. As the number of comments multiplies, so does the number of independent discussion threads and the optimal number of threads is exceeded.
5. (Addended) As one (early) commentator notes below, the simple fact of diminishing marginal utility.
Might some of these mechanisms also help explain why a) history of thought is "ghettoized" as a field, and b) there is such a high premium to working in hot, new fields? The general point is that there are increasing returns to scale for high quality discussions; furthermore those quality discussions are quite fragile and require cultivation and subsidization through norms. Freshness matters, so stale topics will indeed encounter discrimination.
Comments are open, who wants to go first?