The major issue that I have is the erratic handling of the Danish language specific characters æ ø å.
I can send the same message to two different mailings lists and get different results and within the same message can the rendering be different in the subject and the body.
The other two are more general improvements:
The ability to "park" a message (as in TheBat!) to prevent accidental deletion
and
a smooth scrolling function in the message body as Mozilla's SmoothWheel add-on.