We changed the way threads are marked as being read to hopefully be more accurate so that you don't miss out on any new posts if you come to the site and leave. This should be better than the current cookie/inactivity thread marking system.
What this means for you: Your first visit back to the site a lot of threads may display as unread even though you have already read them. To mark all threads as read, go to "Quick Links -> Mark Forums Read." Hopefully this slight inconvenience is outweighed by the benefit of the new thread marking system.