New in Labs: Mark unread from here
I subscribe to a lot of really active mailing lists. Oftentimes, an email in my inbox will get dozens of replies before I get a chance to start reading it. If I only have a few moments to look at a particularly long and interesting discussion, I'd like to start reading it then; later, when I have some more time, I'll pick up where I left off. However, if I mark the thread as unread, it will return to its previously read state without updating to show what I just read. When I come back to it, I'll have to search for the last thing I remember reading. If I star the first message I still want to read instead, I might not remember that it needs to be read when I'm in my inbox later (and when I do read it, I'll have to expand lots of messages before I can read the conversation properly).
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. Give it a try and share your thoughts.