It would be very nice if you could return to a specific post and determine if the addressee had returned to read the response. I am aware that you can tell if your PMs have been read but not messages here.
I expect that this might be a fair sized project for Lithium but that's what they do 
improve their product.
This feature might also be useful when there is a need to limit responses to just the addressee.
[or maybe I just have too much time on my hands]