Can't believe I've never used the "new replies to your post" function! But trying it now it's bringing up 25 pages of "updated topics" going back to February 2010, which was shortly after I joined MW, so how is it actually supposed to work? Also it's showing the last posts from people today on threads I haven't been on for weeks but which I know have been quite active.
It brings up any replies you haven't read yet to any threads you've posted in,so you don't have to go and check if someone has written in them after you.It's only for ones you've posted in though,it doesn't alert you to any new threads that have been replied to or anything.
Of course,with you being here for such a while now,you'd have MILLIONS of unread replies coming up if you've never checked them!(Once you do,they're marked as read,and the thread doesn't show up again in the unread replies section until/unless someone else posts in it.)If you want to use it,but don't want to wade through all those threads,there is a "mark all messages as read" option you might have noticed,which would let you start off with a clean slate.