Via Jeff Atwood, San Francisco-based programmer Tantek Çelik's definition of Email as "Efail:"
All forms of communication where you have to expend time and energy on communicating with a specific person (anything that has a notion of "To" in the interface that you have to fill in) are doomed to fail at some limit. If you are really good you might be able to respond to dozens (some claim hundreds) of individual emails a day but at some point you will simply be spending all your time writing email rather than actually "working" on any thing in particular (next-actions or projects, e.g. coding, authoring, drawing, enjoying your life etc.) and will thus experience a productivity failure. The obvious solution is to push as much 1:1 communication into 1:many or 1:all forms such as public blogs and wikis. ...
think two specific reasons in combination account for most of the problem. [First,] point to point communications do not scale. ...
The second reason that I think email is becoming a worse and worse problem is directly due to its higher usability barrier, that is: Emails tend to be bloated with too many details and different topics.