>But consider: in the 30 odd years of GUI prevalence, there has never been an example of a GUI email client as simple (for some value of simple), functional, and powerful-for-power-users, as Mutt and Pine.
That's exactly my point though - why do most email clients suck so bad? Slow and bloated are the most common adjectives thrown around (Outlook, Thunderbird), and if not that, then there's the weird UI's (Eudora, The Bat) - and then there's the terminal clients, while very nice, very fast, very functional, completely ignore all progress made in user interfaces over the past 20 years. (Pine, Mutt)
It's like you have a choice: Modern, Fast, Functional. Pick any two.
This is something I can imagine coding . Give me something like Pine or Mutt that works in a GUI and can render HTML and images, but still "looks" and runs like Pine or Mutt. Imagine it in your head for a moment - the same UI, but instead of html strewn about everywhere, you have actual rendering of html and images (via choose-your-favorite engine, leaning towards Webkit) in the same window.
I don't think that HTML rendering, speed, and functionality are mutually exclusive goals.