1. It is unstructured. Unlike a formal report, an Email has no context, no index, no contents table.
2. It has no pattern. Some like to dive right in, others like to create a long preamble - you just never know what to skim and what to absorb
3. It is often frivolous. You tend to get involved in conversations and issues that you would normally not consider worth your time.
4. It is self-documenting, which makes people defensive and afraid of saying what they really mean
5. It makes you think you're doing real work - when you are in fact just having the equivalent of a dozen simultaneous water-cooler discussions