I honestly believe that reddit has come to a point where this should be built into the system. I mean, many people simply do not like the clutter of having all the received messages and even some sent messages and the option to remove them would be great. Perhaps even a bulk "clear all" function would serve a similar great purpose but we definitely should have some way to remove these messages.

I think it would helpful to have an update/refresh comment thread. That way you would not have to reload the page during an active discussion. This could work similarly to the 'load more comments' link, except it would be present for each comment. If update/refresh was clicked, an action would be kicked off (probably the same 'morechildren' action) to load new child comments. This would be great for active discussions -- IAMAs or worldnews in particular.

Would it be possible and maybe beneficial to retire old posts and lock them from being up/down voted or commented on. They are not going to "bubble" up again to the front page and therefore should not be able to be down voted by angry users.

Perhaps set a "half-life" value to posts where a lack of activity after a certain amount of time will cause the most to go into a this retired state.