Terseness can be good sometimes; I have a tendency to write really long emails, even when doing relatively trivial tasks like customer support or dealing with a salestard. The sales people are all, as the kids would say, "Tl;dr" then they present me their standard product. Irritating, as that behavior is trivially replaced by a perl script.

On the other hand, conveying actual information is also good sometimes. If someone asks me a sytem administration question and I have a few minutes, they usually get a pretty useful answer. I know several system problems at prgmr.com were solved by a customer because when s/he complained, I gave a full description of the actual problem and the customer was able to help me solve it.

So yeah, it depends on what you are doing, and the literacy level (and interest) of the recipient. Email can be short, when appropriate, and learning how to be to the point is a very good skill, but email can also be long and detailed when you need that.