Here’s a hidden new feature in Outlook 2013 that you may have missed. You can easily switch between looking at all emails in a folder and just the unread emails by clicking on the icon on the top of the mail list.
This feature is not available in Outlook 2010 and earlier.
Some people use Unread emails to indicate that an email still needs action. If you do this then you may also be interested in the following article.
I don’t do this myself. Any email that is in my Inbox still needs some work.
If you work in a similar way to me, I recommend looking at eeminders for Outlook. eeminders helps to keep the Inbox uncluttered by removing emails out and then bringing them back only when I need to work on them.