Just working my way through GTD and find myself nodding in vigorous agreement with David's passion for keyboard shortcuts. As a two-fingered typist, I can't afford to lose half my pecking power grabbing for the mouse.
Rarely do I ever reach for the mouse when e-mailing, for example.
CTRL+SHIFT+M = New message
CTRL + K = Check names
CTRL+SHIFT+B = Address book
TAB = Move to next field
SHIFT+ TAB move to previous field
CTRL+ENTER = Send