a set of extensions built on top of .NET and integrated with WinForms to provide application-level services in the same flavor as the Microsoft Foundation Classes (MFC)
From Jakob Nielsen, usability guru. The user's experience should be similar to all other similar applications to minimize learning curve and maximize usability.
Mac OS X, which combines a powerful core foundation with a compelling user interface called Aqua. With advanced features and an aesthetically refined use of color, transparency, and animation.