The issue with that is that you would probably get people starting a thread to state some particular view and then simply deleting everything that disagreed with them.
I remember this happening a lot in IRC with ban-happy ops and on smaller forums. Eventually either everyone gets fed up and leaves or it just becomes an echo chamber.
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.
I remember this happening a lot in IRC with ban-happy ops and on smaller forums. Eventually either everyone gets fed up and leaves or it just becomes an echo chamber.