The following is is a posting from the Official Gmail News Blog:
Posted by David de Kloet, Software Engineer
Gmail’s threaded conversations are useful because they keep your messages in context — you don’t have to look for previous messages to see what people are talking about. But sometimes, when many people are replying to the same conversation, you open a conversation and quickly wish you hadn’t because you don’t have time to read all the new messages right then.
You could mark the conversation as unread, but this makes all the messages in that conversation unread. And the next time you open the conversation you have to remember which messages you already read and which ones you didn’t get to yet. Since we at Google send and receive a lot of email, we found this pretty annoying. So we decided to fix it.
Now if you’re reading a conversation that had unread messages when you opened it and you mark it unread, Gmail will only mark those messages that were unread when you opened the conversation in the first place. It’s a small change, but it’s the little things that can make a UI feel right or wrong, and we hope this makes Gmail a little bit more right.
Though we realize this change would also be useful when reading mail on your phone (people mark stuff unread on their phones so they’ll remember to come back to it on their computers), for now it only works on your desktop browser.
A small (but helpful) change to ‘mark as unread’