I think Tom that this maybe begs the question. The enquiry as to "the" difference between the two suggest that perhaps there is some essential distinction.
Whereas maybe there are differences but they may not necessarily be in the essence, but in the periphery, as it were.
It seems to me that this is a discussion group, but that since it retains its own history, it is more amenable to pondering the subtleties and nuances of individual words over a longer time, and of referring back to previous exclamations with somewhat more authority than a "normal" conversation usually supplies.
It is an aid to this pondering that the words used and their context are more or less freely available, all the time, to anyone.
Further, I would say another difference is in the fact that in a typical discussion group (in which people physically meet) any reference material used is typically found in external sources and media. Whereas here, both the discussion and a great proportion of reference material used by participants are found in the same medium, and so the conversation has a more seamless quality about it that does offer some perhaps unique differences.
For one thing, the use of material out of context to make spurious points is much harder to get away with here.
For another, this is a public conversation, and not only can anyone join in, but there is a corresponding lack of any assumption of the participants being "representative" in any way, a perfidious notion that seems to me to infest much of the rationale behind the use of discussion groups and focus groups in marketing. They are not assumed to speak with authority on anything other than themselves.
I'm not entirely sure that these distinctions "matter" at all.
They matter to me because this is something I have both personal and professional interest in. Ilike the anarchic egalitarian qualities of blogging. I've worked in the telecommunicatiosn industry for over 16 years so I have a professional interest in all net phenomena. I have both personal and professional interest in marketing theory and practice.
To your traditional marketing person perhaps these would be negatives, and might matter only since they seem to threaten the security of many of the assumptions that underly their work?
There is also the aspect that Chris talks about in GM-WWWP, that the people here were drawn here by common interests and passions, not pseudo-randomly selected. This seems most to me like an essential difference.