Touch screen interfaces have become the standard for controlling mobile devices, and they represent one of the most exciting and profitable fields in modern user interface design. For many applications, they engender an ease of use and intuitive tactile response that is unparalleled. For example, the drag, pitch, and zoom gestures that Apple standardized in iPhone app design have made mobile internet browsing competitive with a traditional desktop experience. However, along with the many advantages and possibilities of touch screens come some disadvantages and limitations. Some limitations are obvious in moment-to-moment physical input, and others are best seen in light of the overall presentation and experience of the application.
The reality of touch screens is that they are not as accurate as traditional mouse and physical button-navigated applications. Small interface elements that would be easy to select with a cursor can become frustratingly elusive when navigated with fingers. This is especially true if the user is holding the device with one hand and using the same thumb to operate the application, as is common.
Also, while touch screens enable intuitive new methods of interaction, they often do a poor job of replicating traditional interaction methods that may be unavoidable. Touch screen keyboards, with their small buttons and lack of tactile response, are notoriously inaccurate and unsatisfying to use. Also, keyboards and other kinds of virtual button steal away screen space that would be dedicated purely to the view on a desktop application.
Yet another issue is that interacting with a touch screen involves obscuring the screen itself. While this may not be an issue for some apps, it is a massive problem for apps that must display a lot of complex or real-time information. For example, this is one of the biggest interface design issues facing game and GPS-enabled app developers.
Touch gestures, as powerful and intuitive as they may be, represent more challenges for mobile interface developers. For example, if an application demands many different gestures, it may be difficult for the programmers to write routines that will easily decipher which gesture the user is intending to perform. A user who feels that the program is constantly misunderstanding their intentions may quickly become an annoyed user, the kind of user who will sink a program’s App Store rating.
Granted, these problems afflict iPhone design more than iPad design. However, given that most mobile developers cannot afford to focus only on iPad development, such issues demand consideration. Apps that can integrate touch gestures usefully, are designed for quick bursts of interaction, that require only a few kinds of input, and that only need to display small and consistent amounts of information are excellent candidates for touch screen deployment. Programs that require lots of button input, or that need to display large and varied amounts of information, will not move so smoothly into the touch screen app arena and would require more extensive development to ensure they transition to these devices in the most efficient and usable way possible.
Apple have some great guidelines on how best to design for the iPhone and the iPad, and whilst they are not absolute practices which must be adhered to, they do give a great starting point.
Read more at Apple