The funny thing about the art of communication is that most people aren't the least bit artistic. Being a good communicator carries at least two important responsibilities; to communicate your point clearly to the other(s); and, to be sure they understand the information they are receiving. Now, this doesn't absolve the listener of the responsibility for intelligent listening. Its a two-way street that means that both parties have to work at it. The listener needs to pay attention to queues other than merely the words. This provides a context for the information being communicated. The tone, inflection, body language, and even how and where the communicator looks. All of this helps set the environment for the listener to receive the information. The person communicating needs to also pay attention to these non-verbal queues to ensure that their information is not misconstrued. These things can potentially tell a listener to take all things seriously or to ignore the information altogether.
This becomes increasingly difficult as we replace face-to-face verbal communications with other forms, such as telephone, e-mail, or even the printed word. You lose a valuable tool when these other forms are involved. You lose body language and, except for telephone, you use inflection and tone.
For informal messages, usually over e-mail or IM, people have taken to using emoticons or embedding items such as (grin) to add the missing elements to the message. This is handy, but not at all appropriate for professional communications. In these cases, it is best to review the message carefully and to remove or clarify anything that does not have a 100% clear context for the reader.