Not quite as entertaining as Shrek, but Dock animation can be an important and useful function in your application. For example, Dock animation is a helpful way to indicate the status of your application.
User Assistance. This is helping the user with the proper "next step" when performing a task. Less guesswork for the user on what to do next makes for a better experience.
For example, if you see an AIM window peeking out from behind your browser and you click on it, that window will come to the front, but the main application window will not. The Mail.app/Activity Viewer is another example. The Aqua system of layers works well in many instances, but not in all. Thank goodness that the Dock is always there to come to the rescue. I know that clicking on an application icon in the Dock will always result in not only the application coming to the front, but also any non-minimized windows associated with it. And if the application is active but no windows are open, clicking on the Dock icon should create a new window in that application.
This is the first thing your users see, and probably the single most important visible part of your application. It is the first chance you have at making an impression and the best chance to help establish your brand.
Adhere to System Appearance. Does your application use all the sweetly colored buttons, delightfully shaded windows, and all the other "bells and whistles?"
If an application is designed well, the reward for users is that they will learn it faster, accomplish their daily tasks more easily, and have fewer questions for the help desk. As a developer of a well-designed application, your returns on that investment are more upgrade revenue, reduced tech support, better reviews, less documentation, and higher customer satisfaction. The rewards of building a good-looking Aqua application are worth taking the extra time.
Adhere to Window Models. Document windows, Utility windows, Click-through, Layering, Drawers, Controls. How do users open windows, how do you properly title windows?
You Must Promise. To call your mother, to help old ladies cross the road, and to turn your cell phone off at the movies.
By building an application that takes advantage of Aqua's many facets, you help ensure that your application will not only look good, but have a chance of becoming a raging success. After a new user clicks on the icon of your program, the first thing he or she sees is the application interface. I know that when I review a product, I am very critical of its visual design. I usually have a short time to learn the new software, so design and ease of use are very important. Aside from those who marvel at the beauty of the command line, most users tend to react the same way.
For my Paint application, I created a series of icons to simulate a rendering algorithm. While the application is performing this CPU-intensive task, you can always see the status of the document by the icon changing in the Dock.