It would be nice to have a complex archive system, so that posts could be rolled to a backup board instead of removed.
It would be useful to have a browsable history that was separate enough to not affect performance, but that users could still mine/search for past posts of interest.
This would be useful when trying to determine if someone has been a problem poster in the past. We currently use the "Move this Thread" feature to move questionable posts to Admin only boards, but it still moves all replies to the post as well.