According to Apple, a great application has the focus on content. Simply, it must be realistic and must take full advantage of device capabilities to enable enhanced interaction. A user interface that is unattractive, convoluted, or illogical can make even a great application seem like a chore to use, so when you design the UI, you must have in mind how your users think, act and work.

Because you are working with a touchscreen device,the users are more engaged with the tasks and they understand way better the results of they actions. You must have in mind that the interface is multi-touch so using gestures will offer a sense of control over the objects that you see onscreen, because you can touch them without using separate controls. One of the best examples of multi-touch is pinch to zoom, instead of tapping.

The control over the actions must be initiated by people, not by applications so even though the app can suggest an action’s flow, the final decision must be made by the user. The best apps find the correct balance between giving people the capabilities they need while helping them avoid dangerous outcomes.

Creating an application – the checklist:

1. List All the Features You Think Users Might Like. Go ahead and brainstorm here. At this point, you’re trying to capture all the tasks related to your main product idea. Don’t worry if your list is long; you’ll narrow it down later.

2. Determine Who Your Users Are. Apart from the likelihood that your users are mobile and that they expect beautiful graphics, simple interactions, and high performance, what distinguishes them? In the context of the app you’re planning, what is most important to your users? Using the grocery-shopping example, you might ask whether your users:

3. Filter the Feature List Through the Audience Definition. If, after deciding on a few audience characteristics, you end up with just a few features, you’re on the right track: Great iOS applications have a laser focus on the task users want to accomplish.

4. Ensure that Universal Apps Run Well on Both iPhone and iPad. If you’re planning to develop an app that runs on iPhone and iPad, you need to adapt your design to each device.

5. Reconsider Web-Based Designs. If you’re coming from the web, you need to make sure that you give people an iOS application experience, not a web experience. Remember, people can visit your website on their iOS-based devices using Safari on iOS.

6. Focus your app. Websites often greet visitors with a large number of tasks and options from which they can choose, but this type of experience does not translate well to iOS apps. iOS users expect an app to do what it advertises, and they want to see useful content immediately.

7. Design for touch. Don’t try to replicate web UI design paradigms in your iOS app. Instead, get familiar with the UI elements and patterns of iOS and use them to showcase your content. Web elements you’ll need to re-examine include menus, interactions initiated by hovering, and links.

8. Let people scroll. Most websites take care to display the most important information in the top half of the page where it is seen first (“above the fold”), because people sometimes leave a page when they don’t find what they’re looking for near the top. But on iOS-based devices, scrolling is an easy, expected part of the experience. If you reduce font size or squeeze controls to fit your content into the space of a single device screen, you’re likely to end up with unreadable content and an unusable layout.

9. As much as possible, avoid increasing the user’s cognitive burden. Users are familiar with the appearance and behavior of the standard UI elements, so they don’t have to stop and think about how to use them. When faced with elements that do not look or behave at all like standard ones, users lose the advantage of their prior experience. Unless your unique elements make performing the task easier, users might dislike being forced to learn new procedures that don’t transfer to any other apps.

10. Always defer to the content. Because the standard elements are so familiar, they don’t compete with the content for people’s attention. As you customize your UI, take care to ensure that it does not overshadow the content people care about. For example, if your app allows people to watch videos, you might choose to design custom playback controls. But whether you use custom or standard playback controls is less important than whether the controls fade out after the user begins watching the video and reappear with a tap.

11. Think twice before you redesign a standard control. If you plan on doing more than customizing a standard control, make sure your redesigned control provides as much information as the standard one. For example, if you create a button that doesn’t have the rounded, dimensional appearance people associate with buttons, people might not realize that it’s tappable. Or, if you create a switch control that does not show where the opposite value is, people might not realize that it’s a two-state control.

The checklist above is an abstract of the Apple’s official guide. For the full stuff, you should check out iOS Human Interface Guidelines.

Below you can see some great examples of iPad UI’s.

Tiny Gift

iMEData iPad Application

MediaMonks

Ipad e-learning App

Citibank Exec iPad News Reader

Map Ally

Allergan’s Health Care iPad Presentation

RTL Social Viewing App

Spring Ahead & Fall Back

Rookies Guide to Baseball

Scholtès Brand

Amdocs

News24

Awake

Guy Savoy