The Microsoft Word format is a secret code which is proprietary to Microsoft Corporation (a private, for-profit company). The recipient must purchase several Microsoft products (various versions of Microsoft Word itself, plus the Microsoft Windows operating system) just to read your message.
If the recipient of your message already uses Microsoft Word (and a compatible version, at that), then this may not be a problem. However, many people, including myself, choose not to buy or use Microsoft computer software -- we find it of low quality, insufficiently flexible, overly secretive, too expensive, and intentionally incompatible with non-Microsoft software. (The Microsoft Word format is also prime way of transmitting computer viruses.) By sending us Microsoft Word attachments, you are implicitly trying to "force" us to purchase Microsoft products that we do not want.
The situation is analogous to your writing me a note, and then leaving it for me in Antarctica. If I am already in Antarctica then this is fine; but if I'm not, then you are asking me to make a special, costly trip just to read your message.
So, please do not send Microsoft Word attachments. Do not try to force the world to give money to Microsoft; they have enough money as it is. Instead, send your messages in a non-proprietary format. The best option is plain-text format: simply copy-and-paste the text of your document(s) directly into your usual e-mail software, as an outgoing plain-text e-mail message. Or, if more complicated formatting really is necessary, then there are plenty of sophisticated but publicly-readable formats available, including postscript, pdf, html, rtf, and tex/latex/dvi.
Just think: Every time you send a Microsoft Word attachment, you are telling the world they have to give money and control to the Microsoft Corporation just to read your message. But every time you send a plain-text (or other publicly-readable format) message, you are telling the world that you support their freedom to use their computer resources as they see fit. To paraphrase Marshall McLuhan: The file format is the message.
So, make the change today!
Sincerely,
-- Jeffrey S. Rosenthal, Professor, Department of Statistics, University of Toronto