I never read HTML emails. All my email clients are configured to display only the text version of messages. Please don't send HTML emails as I will not see them. I always considered HTML emails annoying; nowadays however everybody and their friends seem to just take the default settings of their email clients and happily send HTML away. Most of them get to be unreadable anyway, but the frequency of these is an annoyance and also an ever-present danger. Other people said it better so I am not going to rehearse the arguments here.
Since I am at it, whenever you attach documents please do not use proprietary formats (such as Microsoft Word). Unless I know where it comes from and I really need to see it, I will never open an email attachment that is not written in some generic format such as PDF. The underlying reasons for this are again said better by other people but on top of this I also like to mention one personal reason: Whenever I need to read a Word document I have to launch a big application (OpenOffice) which wastes more than ten seconds of my life; a PDF or Postscript document opens almost instantaneously on my system.
Of course, email has been invented to transport plain text, so ultimately that's how I like it the most. Most of the time one does not need anything else anyway. I have seen (and fail to understand) people writing a piece of text and then attaching it as a Word (or for that matter PDF or anything else) document instead of pasting it into the email message. I have also seen (and still fail to understand) people writing a plain text message as HTML simply because their signature contains formatted text and images. I consider all of this bad practice and I really dislike it.
() ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments