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

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.