I've never seen anyone show what earth-shattering advantages HTML brings to mailing lists, and since almost all of the features that HTML email provides can be provided by simply including a URL to something else, they can scarcely be said to outweigh the possibility that some users of the list might be annoyed or inconvenienced because their mail client/screen reader or whatever can only read "normal" e-mail.
There are all sorts of things one can attach to an email which would make it "richer", but which require parsing/unmunging/opening outside a mail client - heck, Blender email would probably be loads of fun, as might Flash or Silverlight email if you liked that sort of thing, but you still wouldn't expect everyone else to be thrilled at receiving it.
In the end, I'm glad this is a text-only mailing list, and I'd be happy if we could stick with the convention of sending proper email (or in my case *trying* to send proper email using the gmail web interface). It's a *good* convention that makes sense.
Marcus