You know, I had this massive post all typed up arguing why HTML email isn't inherently evil. Seriously, it was approaching 2000 words. Then I realized something- I'm over complicating the issue. Everything I was trying to say in the post, can be summed up in this video, showing off Ubiquity from Mozilla Labs. Basically, what you are about to see in a couple examples in the video, you can't do with "plain text" email. This is a video for Ubiquity, which does a lot more, but I think you'll get the idea.
Now, here's the funny thing. For those arguing the case for plain text email- you are explicitly putting yourself in a "plain text world". When someone sends you something, say a mashup of a restaurant review, and you can't view it with mutt, who's problem is that? The sender, or the receiver? Think about that for a second. It's 2009. We should be able to do so much more with email than we're currently doing, as Aza states, but the plain text folks aren't interested in that. "Gimme the text, remove the bloat. The web belongs on the web." they'll say. Well, I guess the world is interested in passing them by.
Full disclosure- I have been a heavy plain text email advocate in the past. Post 0 and post 1 demonstrate that. Let's just say I've had a change of heart. I want to do more with my email. Also, I'll be sending all my emails in both plain text and HTML encoding, for those who insist on living in the past as well as for those who actually want to enjoy their email.