Boris Anthony is one of those people who is always thinking about how Mail.app could be better.
In the past, he has wondered about how to Mail could be better integrated with iCal 
and how Spotlight searches in Mail could be smarter.
Today he wishes that his smart mailboxes were smarter. At least, that they would tell him the number of emails in each folder, not just the number of unread ones.
And he mocks up an image of what that would look like.
Hey, thanks! :)
As usual, the comments thread there provides further great ideas from others.