OK let me start out by saying that in my opinion SharePoint 2013 is awesome, But….
One of the things I do miss from SharePoint 2010 is the “Sign in as a different users”
This was particularly useful when testing out different permission levels or functions so very sad to see it go. But thanks to Adrian Edgar we do not have to miss this anymore. In the address bar just put
http://<siteurl>/_layouts/closeConnection.aspx?loginasanotheruser=true and you can then just sign out and in again, if you use this a lot my advice would be to create a shortcut on the page, below you can see an example of our icon bar on SharePoint 2013 with the change user icon.