I don't much like the idea of private messaging, but I really don't want to be explicitly notified of anything that has been typed by someone who I have not specifically designated as a friend.
I have enough spam and distraction in my life.
If I did this, I'd make it work like comments work now. In fact, these messages would literally be comments, with some extra flag set.
I take that these comments would only appear to the poster? This means new users won't see these moderating comments / suggestions, hence may miss out on this education until it happens to them. How hard would it be to add a flag to each user's profile so that they will see these comments until their karma reaches a certain level (50?) and then they get the option to turn them off.