This is not a coding tutorial, but nonetheless very important aspect of iPhone app design.
There is a lot more to making an iPhone app than just making a software that functions. For your app to shine, you need to consider a few features that are essential to a good app. As an app developer, you need to think differently than a software developer. An app developer develops and create software that has the nearest human interactions to a user. Our aim, should be, therefore to make a an app that simulates a pet. A pet owner loves his pet, caresses it, loves it, brings it to the bed. That's what you should aim your app should be, and in order to create something like that, consider the following:
1. Sound effects. Many apps out there do not really need sound effects. But sound effects add interaction values to the users. For example, take the Twitter app, it does not NEED sound, but it has some sound like when you drag down the list to update. (It emulates a pet barking or meowing to the owner)
2. Gestures. Do not rely 100% on button taps if you can. Use gestures - pinch, swipe, drag, shake, turn, twirl. (These actions relate to caressing a pet).
3. Help information. Many apps do not have this. Even when they have it, it is too obscured on how to get it. Help info includes how to use the app, explanation of how to use a certain feature that is not so obvious, and so on. Sure, you wrote about it in the app description, but when the user is done downloading your app, do you really expect him/her to return to your app page? No!
4. Animations. Spend a little time to code nice animations. Don't just make things appear out of nowhere. Make it SLIDE, make it FADE, make it DISSOLVE, etc. (These relates to a pet doing tricks for the owner).
5. Polished Graphics. Would you buy an ugly dog or cat as a pet. There you go. Firstly, Apple and Superb Graphics rhymes. So, polish your graphics again, and again, and again.
Well, that's 5 very important aspect of iPhone design. Remember them when you are designing an iPhone app. Good luck.