Then you end up with the case where people only want to reply to threads from certain members who then end up as defacto site moderators. You would also get a l

Then you end up with the case where people only want to reply to threads from certain members who then end up as defacto site moderators. You would also get a lot of troll threads where someone would start a discussion, wait for people to get involved and then just nuke the whole thread to piss people off. People who are good at even-handed moderation in the pursuit of constructive discussion are actually fairly rare; it's a skill most people don't posses as they let emotion and bias get in the way. I've seen a number of online communities combust over the years due to either neglect or infighting, the few that remain do so because they have a strong moderation team with a clear common goal. I think the answer is to separate discussion sites from other sites which have a different goal. Stack overflow is an example of this.