HTML Email, just say NO.
It's very obvious you've never used an opt-in email list to build and grow a commercial business.
You think Groupon (and the ilk), or Newegg, etc, would see the huge returns from email they do if they were sending out plaintext?
The only thing that really bothers me about your posts, though, is that you try to say things with such a tone of authority. But you just don't know what you're talking about.
I'm against over-using formatting, but just because some websites are ugly it doesn't mean that the web in general doesn't benefit from being marked up for presentation.
This plain-text only crusade is a sad remnant from a time when the individual bandwidth consumed by each message actually mattered and/or there was a real risk that some people for valid reasons had an e-mail client incapable of displaying HTML emails. Neither is valid any more, and what's more, there's a rich selection of quality e-mail clients available that will happily only show you the plain text version of the e-mail you're looking at: Be liberal in what you accept and conservative in what you send.
I don't think this site is advocating spamming at all, nor do services designed to facilitate sending HTML emails (MailChimp, Campaign Monitor, etc.)
I do like pleasant stuff. Please, if you want me to read your email, don't send me plain text emails.
It's only when I don't know you and you're trying to sell me something that you need to make the email visually appealing.
(not much can be said for badly designed marketing messages though)
To give a very practical example, I sign up for newsletters from FontShop so I can be updated about new fonts. The email includes images to show how the fonts look. The utility of these emails would be drastically reduced if they were to use plain text.
Reference: http://www.fontshop.com/blog/newsletters/june2011a/
There's a reason why in the real world, brochures and flyers aren't done up on a typewriter. People are visual creatures and if you're running a startup and marketing products or services and you're not using HTML email, you're doing a disservice to your investors, shareholders, prospects, and customers because the vast majority of people would rather opt-in to receive emails that look good.