You will also notice that if you don't read a thread, and go away, when you come back, that ball is still red. On the vBulletin forum we had, there was a time limit, that if you left, it would first go into a "the following threads have not been updated since your last visit" list, but then would mark the thread as having been read after a while, even if you didn't read it. Here, it will show as having not been read until either you read it, or you click on "mark forums (or topics) read. I like that.