The HIG should recommend that most commonly-used dialogs should be closeable via
the escape key. When I recently used GEdit for some programming work, I noticed
that most dialogs, like the commonly used Go To Line and Find dialogs would not
close when I press escape. Instead I had to use Alt-c, which is much harder to
press. Since I use these dialogs all the time, using escape would ease my work
considerably. I think that the HIG is the right place to mandate this as a
general guideline.