Or in other words what is the effective size of a thread for a constructive discussion?
A political story with 200 comments? Well that will probably send me into "Someone is wrong on the internet mode," and it really won't matter who is participating, I'm less likely to learn something that's worth knowing and more likely to call someone a moron - and there are more appropriate places on the internet to do that.
A discussion where the people commenting are expressing professional opinions, I'm more likely to read through a significant portion of the comments and more likely to scan all or most of it...though I'll probably not pay much attention to one-liners.
Since threads flow where they flow based on participants, and long threads are often long because the topic drifts as the comments become more deeply nested and generally more interesting to those commenting than those reading, I don't feel any obligation toward reading anything.
Then again, I don't think constructive discussion is necessarily the important goal...constructive expression is. What matters is not exactly that you write something in response to what I write, what matters is if you do something and writing in response is just one form that the doing might take.
But I'll go through the comments if it's a subject I'm interested in and the comments are constructive or add to the original link. Often I've noticed a pattern where the largest discussions start with a comment along the lines of "This article is wrong because ..." and the reply "<quote from above comment> No, it's not because ...". That's a sign to just close the tab.
I have Greasemonkey and the "Hacker News collapsible comments" script installed:
http://userscripts.org/scripts/show/138037
I do recommend that for an improved HN procrastinating experience.