The user interface for a drawing application is split between the main drawing area and the toolbars, dialog boxes and menus. This makes creating a good user interface a difficult challenge, because there tend to be so many functions spread across multiple categories. It helps if the functions are easily discoverable and intuitive in terms of how they work. Being able to toggle parts of the interface on and off, along with other customisation features, is a distinct advantage.
Workflow is also important. Do you have to keep clicking between user interface elements to