New Gmail Labs Feature: 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, Id like to start reading it then; later, when I have some more time, Ill 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, Ill 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 Im in my inbox later and when I do read it, Ill have to expand lots of messages before I can read the conversation properly
Finally, the long conversations in Gmail will not be so frustrating to keep a tab on.
Although, I wonder if this also affects the unread state through IMAP on the iPhone, since currently, if you mark a conversation as unread in Gmail, it’ll mark all the individual emails on the iPhone’s Mail app.