Another welcome Mail enhancement is the addition of predefined smart mailboxes, such as Unread, All Drafts, and Attachments. You can display any or all of these mailboxes, which show aliases to messages that meet their respective criteria, and you can order these mailboxes in the mailbox pane as you see fit. Now you can see in one place just your unread mail or find all the drafts you had saved across your mail accounts.
You can also add mailbox folders to the mailbox list, saving you the step of drilling to the Accounts section and then navigating to those folders each time. It's great for mail folders you use all the time.