When people can more easily fire off all sorts of messages–from
updates about their breakfast to questions about the evening’s
plans–being able to figure out which messages are truly important, or
even which warrant a response, can be difficult. Information overload
can lead some people to tune out messages altogether.

Such noise makes us even more dependent on technology to help us
communicate. Without software to help filter and organize based on
factors we deem relevant, we’d drown in the deluge.–Jessica E. Vascellaro, Wall Street Journal

The article is more about the rise of microcommunication tools than it is about the end of e-mail, but it does a fair job explaining the difference.