Now you can choose when a message gets officially marked as "read" – either right away, or after 2 seconds or 5 seconds. This gives you a chance to glance at a message in the reading pane and decide if you want to come back to it later.
If you decide you do want to come back to it later (your eyes can learn a lot in just two seconds, and gosh they can dig deep in five), the message will remain in bold in your Inbox. This could very well be the easiest way to keep messages on your radar!