How to mark unread only the messages you want in Gmail?
This new lab feature will show you how to do it.
There's a new feature in Gmail Labs that will help with this. When you enable Mark Unread From Here from the Labs tab under Settings, you'll see a new "Mark unread from here" option in the drop down menu found in the upper right-hand corner of messages.
Clicking this option on a message tells Gmail that you want that message to be the first one you see when you reopen the thread later, with all messages after it open for easy reading. So, when you leave partway through reading a long thread, figuring out where to start reading again is easy.
Official Gmail Blog: New in Labs: Mark unread from here
