Overall I like the idea and most of the design. There are a lot of awesome little touches in it. But a lot of small things seem weird. Maybe some of these can be smoothed out for the full release:

  - The status item (why?) that doesn't have a
    drop-down menu at all, but simply shows/hides the
    application?
  - The slightly-off-feeling fonts and buttons
    (especially labelled buttons like "save" and "send").
  - The hidden-seeming account preferences that you can
    only access by double-clicking.
  - Some sort of custom implementation of a toolbar
    that bypasses the normal configuration option for
    toolbars in OSX apps.
  - No option for plain text composing. I know it's
    minimalist so you might want to minimize options,
    but nothing's more minimalist than plain text.