From Apple's developer guidlines

Introduction to Apple Human Interface Guidelines

Apple has the world’s most advanced operating system, 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, Mac OS X makes computing even easier for new users, while providing the productivity that professional users have come to expect of the Macintosh. The user interface features, behaviors, and appearances deliver a well-organized and cohesive user experience available to all applications developed for Mac OS X.

These guidelines are designed to assist you in developing products that provide Mac OS X users with a consistent visual and behavioral experience across applications and the operating system. Following the guidelines is to your advantage because:

Users will learn your application faster if the interface looks and behaves like applications they’re already familiar with.

Users can accomplish their tasks quickly, because well-designed applications don’t get in the user’s way.

Users with special needs will find your product more accessible.

Your application will have the same modern, elegant appearance as other Mac OS X applications.

Your application will be easier to document, because an intuitive interface and standard behaviors don’t require as much explanation.

Customer support calls will be reduced (for the reasons cited above).

Your application will be easier to localize, because Apple has worked through many localization issues in the Aqua design process.

Media reviews of your product will be more positive; reviewers easily target software that doesn’t look or behave the way “true” Macintosh applications do.

The implementation of Apple’s human interface principles make the Macintosh what it is: intuitive, friendly, elegant, and powerful.