I've been really enjoying sup -- http://sup.rubyforge.org/ -- in fact, that really wordy quote about five down is mine.
Mutt -> Gmail -> Sup ? I hope so.
Btw to be able to write new emails with vim again is a joy... Still almost everything Sup is doing is in theory possible to do in a web-based or resident application with improved font rendering, simple icons to signal status of messages and so on.
Remember xchat? This IRC client had a lesson, it was possible to retain everything of good there was in text-only IRC clients, and add the good stuff of GUIs. The nerd email client I'm waiting for is something like this.
Fast: just scan IMAP for new messages but take everything local otherwise.
Great interface: mixed between curses but with improved rendering and config menus thanks to GUI toolkit or even web-based can be ok.
Easy to configure: unlike mutt for example. Spun is nice from this POV at least for the first steps.
Give me my editor: if resident, fire vim for me in some way, otherwise implement a decent editor with vim and emacs key bindings if it's web based.
It's worth to note that even a web-mail can be scriptable, using javascript.
Btw there is something strange going on here: the textarea is the most popular text editor out there, the most used one where millions of people write short and long text every day, and still browser developers are not realizing that it should be improved.