Orig: http://www.georgedillon.com/web/html_email_is_evil.shtml
The 7 sins
1. HTML e-mail is dangerous

Nearly all viruses are transmitted by email. Both plain text and HTML mail may carry malware attachments but with HTML there is a significantly greater risk since some malware can exploit vulnerabilities in the HTML parser to automatically execute code as soon as the message is viewed in the preview pane (i.e. without the attachment having to be 'opened'.)

2. HTML e-mail wastes bandwidth

Look at the source code of any HTML message and after the headers you'll see the message body is duplicated, once in plain text and once in HTML. So most HTML messages are at least twice as big as plain text only, and they can be many time larger.

3. HTML e-mail doesn't always work

Some popular e-mail readers (e.g. Pegasus) simply don't read HTML mail, others (Pocomail and even AOL) have difficulties displaying it properly.

4. HTML e-mail can connect to the internet by itself

If you're off-line, opening an HTML email cantaining images may (by default) open a connection to the internet.
5. HTML e-mail renders slowly

Some mail apps (e.g. Outlook) can slow down considerably when rendering HTML. The need for an HTML parser has also led to code-bloat in email apps generally.

6. HTML e-mail is not always reader-friendly

HTML allows the sender to use unreadably small or non-standard fonts, clashing colours, badly formatted images and sometimes there is no quick or easy way for the reader to adjust the appearance to THEIR choice.

7. Digested lists hate HTML mail

Subscriber lists, particularly those with a digest, discourage and sometimes block HTML (since it appears in the digest as a mess of code).