"Wow, someone commented ... nobody ever does that."There's several reasons for that I'm coming to find out. The first is that readers are just not used to joining in on a conversation. They're just there to read, get the information, and move on. One does not usually pick up a pen after reading something in the Wall Street Journal, Business Week, or Wired and shoot off a letter to the author. (Actually it turns out that authors in these magazines love hearing from readers, and will almost always respond to comments or questions sent to them.) So maybe its part conditioning, part laziness, and part mindset.
Even if you do get past that and decide that you have something to say, leaving it as a comment is not necessarily your first choice. Since the point of web logs is to give each person their own voice on the web, you may want to to continue the conversation going on at your own site. Or even if you're thinking of leaving the comment at another location, you wonder if anyone will ever read it. (With RSS feeds, often the comments are not conveyed; and on some sites I have to wonder if the log author will even notice the comment.)
Even when I do decide to post a comment on another site, though, I run into yet another problem: finding it later. Sometimes my comments get pretty long (longer than a good number of my posts), and I'll be thinking a couple weeks later about something I said, then I've got to wonder if I can even find the comment again. And if someone else happens to post a response to my comment, its pretty unlikely that I'll see it unless its on a site I go to very frequently. Don Park just lifted a comment off of another blog and stuck it on his weblog because he wanted to keep the memory. (The post was about losing a pet.)
Maintaining conversations is a problem a lot of web tools have had. Wiki's suffered from this problem, and even old net-news (though with lower bandwidth groups, the later readers with thread stitching made a decent attempt at keeping track of it all). Track-backs try to solve this problem, but are harder to use and with only an excerpt in the source blog, the flow of the conversation is broken up.
I have faith of course, that someone will come up with a crazy idea, and after its implemented, we'll all realize how brilliant it is, and the global consciousness will take another leap forward. For now, we'll put up with partial solutions, and lost conversations.