As explained in the Gmail blog, the really annoying bug of the “mark as unread” button is now fixed.

Before if you had a new post in a really old thread with dozens of mails and marked it unread, all the messages in the thread were marked as unread, expanding them all ! It was really annoying as for long threads it meant scrolling a lot to find the last mail really unread message.

Good job, but now a “mark all messages after this one as unread” option on each message seem needed.