Simplicity in analysis, design and implementation must be the hallmark of your application. It must exhibit simplicity in API, tools and usability. You must give ample space to your user interface so that the user can breathe. You must provide a stable application programming interface such that the kernel may change internally without breaking compatibility. Avoid clutter.
To paraphrase Antoine de Saint-Exupery,
Design is achieved not when there is nothing left to add, but when there is nothing left to take away.