I was recently going through a nice list of Gmail Tips maintained by Jim Barr, it has a lot of useful tips out of which tip 25 tells you how you can create a label named "Unread" which automatically displays all the mails which are not yet read by you (not just in inbox or just in one label, but in your full email account). But when I tried creating such a label gmail didn't allow me, so I tried creating a label named "Unread2" and then tried renaming it to "Unread" that's when I got a message saying "System specific label names are not allowed. Please try another label".
Workaround for this problem at present would be to just type "label:Unread" in your gmail search box and hit enter. This will give you the complete list of emails which you have not read till now.
It would be a lot convenient if Gmail guys add such a link near the system folders / labels (I mean near Inbox, Sent Mail).