It’s very simple. No unread item count.
WordPress Reader displays new site updates in a stream (or as dave Winer prefers a river). It’s just like Tumblr’s Dashboard for RSS with no indication as what you have and have not read although when you’re in Reader you are notified of new posts. But never about posts you may have missed.
This is just how it should be. Unread item counts, like unread email counts are the work of the devil and have no place within streams of information.