The conflict between using the screen as input versus output is the driving problem, but you also have to deal with the inaccuracy of touch input and the conflict between a comfortable holding angle and using the accelerometer for input.
A good game design will consider these factors when deciding exactly what the goal of game play is and how the user must interact with the device to accomplish that goal.
For instance, although an FPS game on the PC may use the pinpoint accuracy of the mouse and keyboard to its advantage, porting to the iPhone would require a change in design. Rather than expecting accurate aiming from the player, it should focus more on rewarding the player for being in the right place at the right time—perhaps by automatically correcting the player’s aim within the current view angle.
EA’s SimCity for the iPhone is an excellent example of adapting a complicated user input design to the iPhone. The original game requires a keyboard and mouse, expecting pinpoint selection of squares on a grid, as well as click-dragging for multiple selections. There is also a heads-up display (HUD) with a slew of buttons for various macro-management features.
In the SimCity port to the iPhone, the designers increased the size of all HUD elements so they are touchable. They simplified submenus and split them into multiple pages to account for the new larger buttons. With the understanding that the player’s interaction with the touch screen can sometimes be inaccurate, they split click and drag operations into multiple steps with confirmation screens to make sure the player intended the exact input that was given.
Due to the specific challenges of input on the iPhone, games should try to simplify the interaction with the player and limit the penalty for erroneous input.
Learn more about this topic from iPhone Game Development.
What do you need to know to create a game for the iPhone? Even if you've already built some iPhone applications, developing games using iPhone's gestural interface and limited screen layout requires new skills. Loaded with descriptive examples and clear explanations, iPhone Game Development provides everything from game development basics and iPhone programming fundamentals to guidelines for dealing with special graphics and audio needs, creating in-game physics, and much more.