The karma system motivates people not to bring up things not already known to the other members. People will be less interested, and more likely to ding them for "not being relevant" (or because they misunderstand it). For instance, the recent post on E-Prime was relevant; very short; interesting; and even practical. Yet it's still sitting there at 0.

I've noticed that very smart people often go to great effort to spend time with other very smart people; and then, instead of listening to them, try to talk as much as they can. Which defeats much of the purpose of spending time with very smart people. This indicates that the motivation for joining groups is as much to impress them, as to learn from them.

It would be very surprising if groups did not spend most of their time discussing things most of them already know. That would mean that the groups consisted mainly of people who were interested in a subject, yet uninformed about it.

<ADDED> If one person points out a previously-unknown implication of mutually-known facts to the group, does that still count as discussing the information shared by members? </ADDED>