Gmail has been all about improving usability lately. Most recently they’ve announced a big improvement in this area fixing a problem that has annoyed countless emailers including Googlers themselves.
You know how when you get more than one conversation going based on one message, you don’t always have time to read each new one? It can get pretty confusing when trying to remember which ones still need to be read when you go back to them. If you mark the whole thing as unread, then you have to wade through the ones you’ve already read.