Design Specs: See units in Percent Values
The ability to see dimensions and measurements of objects in percent.
Shahar Naor commented
Common people, this is 3.5 years old. Cannot believe this has so few requests. Relative size is a must if you really want to support proper design hand-off.
This is so handy in other Adobe products and I need all the time if screenshotting on retina to paste on a non-retina monitor. My initial paste is double the size so simply typing in 50% Enter/Return would be magic!
Vlad Iepure commented
Ability to resize artwork in percentages, relative to its own size or the artboard.