New interfaces always demand extra thought and attention of your audience, and you shouldn't take that lightly. Don't be different just to be different; be different when you believe you can be better. Before you shake free of the standard iPhone interface metaphors to create your own, know the warning signs for when you're inventing something that looks an awful lot like a wheel:
Are you trying to solve a problem you could solve with built-in tools? Favor the standard controls instead of creating something new to perform exactly the same function. (Don't craft web-style tab navigation, for example, when a tab bar, table index, or segmented control will do the job.)
Are you being too clever? A great interface makes the app easy and evident to use. Avoid abstract metaphors, and rely on proven, real-world affordances. Tapworthy apps encourage direct, unmediated manipulation of content: tap a content element to read it (or view it or listen to it), drag it to move it, poke it in the belly to make it giggle. This "be direct" mantra applies to the words and icons of your interface, too. Bank of Mom, for example, is a great little app to help parents manage kids' allowances, but it's tarnished by a puzzling gardening metaphor. The app refers to kids as "saplings" and uses a leaf icon for financial interest and a fluorescent lightbulb for activities. Huh? Weed out those tangled metaphors.
Is your interface metaphor appropriate to the device? Your app should feel like an iPhone app. Your new-to-the-world interface should blend in with the visuals and metaphors of the apps around it. In particular, be cautious about importing interface elements from other platforms, particularly desktop computers. For example, limited screen space means that the windows of desktop platforms don't work well on the iPhone; but even more important, they don't jibe with the sliding screens that iPhone users are accustomed to. Don't use windows or other metaphors that go against the grain of established iPhone conventions.
Got more interface than you need? As you create new controls and interface elements, constantly pare them down to the simplest possible presentation. That doesn't mean you should strip out the personality or take the bot out of Convertbot, but rather that you should remove unnecessary steps, distracting animations, and extra buttons to keep the flow as easy and obvious as possible. In the early designs for image enlargement in the USA Today app, for example, the image included an X button to close it and return to the article. An artifact of desktop software windows, the button was unnecessary on the iPhone, creating the expectation that you had to tap the button directly to dismiss the image. The designers removed the button to allow users to tap anywhere to get rid of the image, a faster and equally intuitive solution.
