The sheer number of apps available for the iPhone or other i-device means that users will only give your app a minimum trial at best before making a judgment. Because users are able to quickly jump from one task to another on the iPhone or other i-device, the time you have for first engaging the user is incredibly minute — but significant.
To avoid any faux-pas that might cause the user to exit your app, let's take a brief look into how users will interact with your app in the short “half-second window,” and then how to make your initial interactions more engaging.
Touch screen users currently know how to swipe, tap, pinch, and (only because it's been bundled with iPhones and relatives) tilt and shake. If your app is going to require any user input other than these actions, be aware that it might present a barrier to your customer's engagement with the app.
Without getting into the math, Fitt's Law states that the time to rapidly move to a target is a function of the distance to the target, as well as its size. In other words, ensure that frequent actions by the user are easier to accomplish.
How would you go about doing this? Combining Fitt's Law, with what is known about user tendencies, it is possible to formulate the following dos and don'ts: