Robert_Charlton - 8:01 pm on Sep 24, 2012 (gmt 0)
Perhaps the feature hasn't been highly cited enough. I don't remember having noticed it before, but I've found a few references to it on the web dating back to May, 2011.
Here's an undated article in Google News Support, which includes a comment about what the feature signifies...
Google News expandable story boxes
Highly Cited: the article that appears to be most frequently. referenced by other articles in this story.
The Google News Support article also notes a few features apparently related to sentiment analysis (opinion, satire, in-depth), and those do ring a bell, making me believe we'd discussed that here*. I'm not sure whether "opinion", eg, is so identified because Google's analyzed it by content, or simply because it's labeled as opinion within the news publication.
* Google "News" is a hard topic to search for in this forum because of the name of the forum.