Apple's Human Interface Guidelines for the iPad outline how to create user interfaces optimized for the iPad device. According to Apple, the best iPad applications: downplay application UI so that the focus is on content; present content in beautiful, often realistic ways; and take full advantage of device capabilities to enable enhanced interaction.
The overview of iPad user experience guidelines listed below is © 2010 Apple Inc. More details on these guidelines and further information on developing for the iPad can be found in Apple's Human Interface Guidelines for the iPad.
Support All Orientations
Your application should encourage people to interact with iPad from any side by providing a great experience in all orientations. The reason is that people don’t view the device as having a default orientation, because they don’t pay much attention to the minimal device frame and they’re unconcerned with the location of the Home button.
Enhance Interactivity (Don’t Just Add Features)
The best iPad applications give people innovative ways to interact with content while they perform a clearly defined, finite task. Resist the temptation to fill the large screen with features that are not directly related to the main task. In particular, you should not view the large iPad screen as an invitation to bring back all the functionality you pruned from your iPhone application.
Flatten Your Information Hierarchy
Although you don’t want to pack too much information into one screen, you also want to prevent people from feeling that they must visit many different screens to find what they want. In general, focus the main screen on the primary content and provide additional information or tools in an auxiliary view, such as a popover.
Reduce Full-Screen Transitions
Instead of swapping in a whole new screen when some embedded information changes, update only the areas of the user interface that need it. When you perform fewer full-screen transitions, your application has greater visual stability, which helps people keep track of where they are in their task.
Enable Collaboration and Connectedness
Think of ways people might want to use your application with others. Expand your thinking to include both the physical sharing of a single device and the virtual sharing of data.
Add Physicality and Heightened Realism
Whenever possible, add a realistic, physical dimension to your application. The more true to life your application looks and behaves, the easier it is for people to understand how it works and the more they enjoy using it.
Delight People with Stunning Graphics
The high-resolution iPad screen supports rich, beautiful, engaging graphics that draw people into an application and make the simplest task rewarding.
De-emphasize User Interface Controls
Help people focus on the content by designing your application UI as a subtle frame for the information they’re interested in. Downplay application controls by minimizing their number and prominence. Consider creating custom controls that subtly integrate with your application’s graphical style. In this way, controls are discoverable, but not too conspicuous.
iPad applications should allow people to interact with them in nonlinear ways. Modality prevents this freedom by interrupting people’s workflow and forcing them to choose a particular path.
Rethink Your Lists
Consider a more real-world vision of your application. For example, on iPhone, Contacts is a streamlined list, but on iPad, Contacts is an address book with a beautifully tangible look and feel.
Consider Multifinger Gestures
The large iPad screen provides great scope for multifinger gestures, including gestures made by more than one person.
Consider Popovers for Some Modal Tasks
If you use modal views to enable self-contained tasks in your iPhone application, you might be able to use popovers instead.
Restrict Complexity in Modal Tasks
People appreciate being able to accomplish a self-contained subtask in a modal view, because the context shift is clear and temporary. But if the subtask is too complex, people can lose sight of the main task they suspended when they entered the modal view.
Downplay File-Handling Operations
Although iPad applications can allow people to create and manipulate files and share them with a computer (when the device is docked), this does not mean that people should have a sense of the file system on iPad.
Ask People to Save Only When Necessary
People should have confidence that their work is always preserved unless they explicitly cancel or delete it. If your application helps people create and edit documents, make sure they do not have to take an explicit save action.
iPad applications should start as quickly as possible so that people can begin using them without delay.
Always Be Prepared to Stop
Like iPhone applications, iPad applications stop when people press the Home button to open another application.
More details on these guidelines and further information on developing for the iPad can be found in Apple's Human Interface Guidelines for the iPad.