The reply links on each message showing who responded to what message are nice if they are used consistetly (otherwise you miss some of the discussion on a subthread of a post - note that this message for example would not show up under yours since i used the button at the end of the thread). I know openacs keeps track of these things underneath if you use the reply to a particular response to a post feature - but that's precisely why I (and this is a personal opinion), and it looks like Don as well from his comment, think its unnecessary complication, db search as well as UI wise. Just becase it can do it doesn't mean its best to do it... and if you have a flat forum view i don't see why you would want threads...
