At Solartech I've had the opportunity to design and implement the GUI desktop application used to control the message signs. The application was written in Java/Swing so that it could share code (and design elements) with the embedded control interface on the LCD/touchscreen of the sign's controller. This project is not yet complete, though substantial progress has been made on it.
My design philosophy for desktop applications is to combine power with ease-of-use by emphasizing the principle that for any task, it should be possible to accomplish that task with only the minimum inherent complexity of the task being necessary for the user to see. Clearly, if you're going to put up a message, you're going to have to type something. But it shouldn't have to be harder than that, though of course the user should be able to make it harder if they have something else that they want to do (e.g. embed the current time in the message). That's how I make an interface both easy and powerful: present the typical task as simply as possible and then give the user convenient ways to detour from it.