Most e-mailed lists

I've been meaning to look into whether there were any consistent criteria for "most e-mailed" articles.  Lots of media sites must have large datasets by now. I'd like to see an analysis of them.

I did find this article from Slate in which some thinking is slapped on the question. It's focused on only one story and the tone is thuddingly cynical (which is saying something coming from me), but it's interesting.