Facebook's redesigned iPhone App (which is outstanding!) features in-line alerts for new content in the newsfeed. The treatment and interface are well done: simple, nonintrusive, yet obvious. Furthermore, it has become a familiar behavior (Twitter.com, Facebook.com and others). I had not noticed, however, that a similar treatment was provided for comments. When viewing a post, comment alerts appear in real-time via the below treatment. Again: it is a simple yet powerful visual. And because it

1) appears in real-time, and 2) is directly related to the piece of content you are reading

It drives users to read the comment, interact and spend more time. Small enhancement that is contextual and powerful: