At present, setting margin and padding on a single element requires a minimum of 24 clicks and keystrokes, as each one has to be opened, edited and closed individually.
It would be a great timesaver to be able to open a dialogue that presented all values at once to be edited by simply tabbing through. An even bigger improvement would be the ability to link horizontal and vertical values - the equivalent of setting
margin: 5px, 10px; or padding: 1em;
margin: 5px 10px 5px 10px; or padding: 1em 1em 1em 1em;
You could also add a keyboard shortcut to the dialogue to make it extra easy :)