Laziness, unorganized behaviour, callousness, however you may want to view this - I had about 3947 unread mails in my Gmail Inbox. I wished to make all of them read without going through each ; Newsletters and weekly digests can be skipped once in a while!. At 100 conversations a page it would have taken me atleast 40 iterations, two clicks (Search for label:unread , Select All and Mark All as read) per iteration to mark all my mails as read. 

Well it would have...if it weren't for Gmail Filters. Heres how you can mark all your mails as read in one shot.

Create a new Gmail filter (Its in your mailbox settings) with is:unread in the Has the words field, then tell Gmail to Mark as read all matching messages. You would get a warning message - Ignore it with an OK. Finally, tick the box next to Also apply filter to conversations below and create the filter.

Once you have done this, navigate to your inbox and there, you are done!

Note 1 : Remember to delete the filter after this operation.
Note 2 : All due credit to Lifehacker.