It seems like there is a tension between these systems as repositories of reference information and as live conversations, with living documents (e.g. FAQs) somewhere in between. You could also think of this as the time dimension, or the distinction between ephemeral and permanant materials.

Ideally, the tools make it easy to transform the best of the living conversations into reference material, and to improve the conversations with easy links back to relevant reference material.

It'd be interesting to list variables, e.g.:
- scale. How many people are in the discussion?
- purpose. What is binding the group discussion together? Share interest in a narrow topic? Common real world experience? A project of limited duration?
- relationship to other media (context). Discussion groups about television shows, IM sessions about what's happening in a con call, wikis supporting a class, etc. Collaboration tools don't have to stand alone.