Purists have long objected to HTML email on aesthetic grounds. On functional grounds, it tempts too many sites to put essential content in embedded (or worse yet, remote) images, thus making the messages not findable via search. For these reasons, among others, Matt Blaze remarked that "I’ve long thought HTML email is the work of the devil". But there are inherent security problems, too (and that, of course, is some of what Matt was referring to). Why?
Although there are no perfect measures for how secure a system is, one commonly used metric is the "attack surface". While handling simple text email is not easy—have you ever read the complete specs for header lines?—it’s a relatively well-understood problem. Web pages, however, are very complex. Worse yet, they can contain references to malicious content, sometimes disguised as ads. They thus have a very large attack surface.
Browsers, of course, have to cope with this, but there are two important defenses. First, most browsers check lists of known-bad web sites and won’t go there without warning you. Second, and most critically, you have a choice—you can only be attacked by a site if you happen to visit it.
With email, you don’t have that choice—the bad stuff comes to you. If your mailer is vulnerable—again, rendering HTML has a large attack surface—simply receiving a malicious email puts you at risk.