Frantically busy in my last two days at Yahoo, but if there's one thing I know about me, it's that if I don't document an idea, I'll forget it, even one as recurring as this one. So:

Copy, Cut, and Paste are great, but three times today and usually at least once a day, I wished there was a Switch command. This command (by keystroke (Cmd-Shift-V) or edit menu) would replace the current selection with the clipboard contents, exactly as paste does, but would also replace the clipboard contents with the otherwise-vaporized current selection.

Another way to look at this is "pick up selection and put down what I was holding before."

Think about this feature today, and see how many times you'd have wanted to use it...