Unfortunately the old Guestbook format was getting so many junk spam posts it was becoming too much work to maintain it. We either needed to make a change or remove the feature completely.
One method to bring back more of the functionality of the Guestbook is to select Show All Topics in the drop down tab near the upper right. This will show all of the subjects ever posted to the Guestbook.
Also, the new Guestbook allows members to reply to posts but anyone can make a post.