I don't like the way 'Mark all as read' works. With NNTP, you could see the forum, see which topics have unread posts, and see the total number of unread posts. If it hasn't been refreshed in a while, you could mark as read, then refresh and see just what's been posted since the previous refresh. What happens here on the web is you lose the chance to view as new any posts that occur between the refreshes. What this means is that you have to reload the page right before marking as read, which has the effect of those posts being lost in the mix of all those posts you aren't interested in.