In most cases, I can't imagine how the GUI should look like.
Like Roberto already mentioned, usability is King while designing any interface. Interface need not be limited to only software. It can be also how a coffee mug handle is designed. Is the end user going to be male? Try and give the handle loop extra height because a mans paw is typical bigger than the female. Is the end user going to be female? She probably likes long nails. It might be a good idea to make the coffee mug coated with some scratch resistant surface. Is the mug going to be used in environments where the user's hands might be sweaty? Give the handle a slip proof grip. And you need to make it look good and attractive.
The basic tenet of usability is minimal learning curve for the end user. Your application might be awesome loaded with tons of super features. But if the end user needs to spend 3 hours learning how to use it, then there is a big chance no one is going to bother unless they have a compelling reason, and even then they will hate it.
When designing your UI, it is a good idea to
Identify who your end users are
Put yourself into the end user's shoes
Approach the UI with a blank mind and try to see if the UI makes sense
Split the UI into logical inter related units
Make the UI intuitive enough so that it's obvious to the user how to use it
Remember a picture speaks a thousand words. Try to use as many icons as possible
And finally, make sure it's good attractive and interesting to the end user.
After you are done with your first draft of UI design, check if there are other applications which are similar to yours in functionality. Check out their UIs and see how they do it. Try to mix and match. Identify areas of improvement in their UIs and try to implement those features in your UI. Do all this even before writing a single line of code. Draw your UI mockup using some tool such as Pencil or a white board or even paper. Show (dont tell) it to a few typical end users and ask them to tell you what it is all about. If they can, you are in business. If not, repeat.