1.  It may not be readable, easily if at all, on other computers, especially by someone who is visually impaired and especially if they are depending on an audible email reader.

2.  HTML increases the risk your email may get kicked by a SPAM filter and not even be seen by the recipient.

3.  Most mailing list servers will not accept HTML and will bounce your message; or, if it delivers the message, will convert it to plain text with undesirable consequences for its content.

Initially, all standard email was sent in — and only in — ASCII text, but it is now possible to send email in HTML.  When you specify fonts and colors in your email client or add graphics to your messages, what you are really sending to your recipient is a web page.  There's no doubt that sending pretty or personalized emails is much more fun for the sender, but most senders are oblivious to the effect of their messages on the recipient, which may not be what was intended.

The most common problem with HTML email is readability, which is primarily a function of text size.  People use monitors that range from 13" to 23" in size and in resolutions ranging from 640x480 to 2048x1536, though the majority are using a 17" monitor at 1024x768 resolution.  While "VGA" resolution (640x480) was once thought dead, the emergence of hand-held email readers has seen a rebirth of this and lower resolutions.  The point is that you cannot assume your recipient will be reading your message on a screen that is anything like the one you are viewing when you create the message.  Nor can you assume that they have eyesight as good (or as bad) as yours.

When you force the appearance of text by setting the font face, size, and color (and the background color), thus creating an HTML message, you may be creating a message that is difficult or even impossible for the recipient to read.  Is it really your intention to annoy your recipient by forcing them to change the resolution of their monitor — or cut and paste your message into Windows Notepad — in order to read it?  Was that remotely the effect you intended?  So how are ASCII text messages any different?

When you send an ASCII text message, no fonts faces, font sizes, or colors are included in the code.  What specifies the visual appearance of the message for the recipient are THE SETTINGS IN THEIR EMAIL CLIENT SOFTWARE.  I emphasize this fact because letting the recipient determine the appearance of the message is so considerate.  You are allowing the recipient to read your message in the font they like and in the size and colors they find most readable.  For visually impaired users, the ability to control the appearance of email message text may not just be esthetic, it may be critical to being able to read it.  And, of course, email readers for the blind cannot read graphics, which means not just pictures, but things like banners and signs.  You don't want to force your recipient to divulge that they are blind when the thing they may enjoy most about being on the internet is not having people always defining them as "that blind person."

Another important reason not to "jazz up" your email is that multiple fonts and colors and, especially, graphics may get the message kicked as SPAM, because those are some of the characteristics SPAM filters looks for.  I used to check my "junk" folder to at least read the filtered message Subjects before deleting them, but I now get too much SPAM to take the time to do so.  If a "real" email gets kicked as SPAM, it's going to get deleted without my having seen it.  I've tried reducing the "aggression" level of my SPAM filter, but when I do, too much SPAM gets through.

Another reason not to send HTML (or "Rich Text") email is that most mailing list servers will accept only plain text.  Or, if they accept HTML, they will convert your message to ASCII text, with the most unexpected consequences.  Often, the HTML code that is in the message, which is not visible in HTML, will become visible after the conversion to ASCII, adding a lot of unintelligble "garbage" to your message. 

Another reason not to use HTML email is the size of the messages.  It takes more code to send email in HTML than it does in ASCII text.  This larger size means messages take longer to download and use up more hard drive space to store.  If you have a high bandwidth connection and are handling only a few messages, the difference may not matter or even be noticeable.  But as a researcher and DNA project administrator, I feel obliged to save most of the messages sent to me, so the cumulative size does matter, at least to me.  And I get so much email, that download time is a factor, even though I have a high bandwidth connection.

Lastly, there's an old dictum that applies here:  "The medium is not the message."  Work on the content of your messages.  Make that interesting.  And stop fussing over the form, especially, stop using form to cover up having spent too little time and effort on the content.  Please.