New Gmail lab feature: Mark unread from here

If you are subscribed to mailing lists with long conversations, then this new lab feature comes in handy:

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.

Read Gmail blog post.