Maureen Dowd: Why did you think the answer to e-mail was a new kind of e-mail?
Biz Stone: With Twitter, it’s as easy to unfollow as it is to follow.


Source: Dowd, M. (2009) To Tweet or Not to Tweet. The New York Times http://www.nytimes.com/2009/04/22/opinion/22dowd.html


This quote captures one of the fundamental advances of Publish and Subscribe over email-like subscription, where you aren't guaranteed an easy, reliable way to opt out of somebody's output.