From Eats, Shoots & Leaves by Lynne Truss:
How often do you hear people complain that e-mails subtract the tone of voice; that it's hard to tell if someone is joking or not? Clicking on "Send" has its limitations as a system of subtle communication. Which is why, of course, people use so many dashes and italics and capitals ("I AM joking!") to compensate.
That's why we came up with the emoticon, the greatest (or most desperate) advance in punctuation since the question mark in the reign of Charlemagne. Emoticons are the proper name for smileys :-)
Forget the idea of selecting the right words in the right order and channeling the reader's attention by means of artful pointing. Just add the right emoticon to your e-mail and everyone will know what self-expressive effect you thought you kind-of had in mind. Anyone interested in punctuation has a dual reason to feel aggrieved about smileys, because not only are they a paltry substitute for expressing oneself properly; they are also designed by people who evidently thought the punctuation marks on the standard keyboard cried out for an ornamental function.
