Principles of iOS app design

A well designed iOS app,

• is consistent in working and looks
• indicates in each screen where the user is and what he could do
• has clear and shallow navigation
• avoid using gestures as much as possible so that the user could understand it by just looking at it
• is discoverable enough so that the user can learn it by exploring it
• sticks to conventions as much as possible and overthrows it wherever required
• has controls that are large enough to be tapped without much effort
• gives clear feedback to user's actions
• feels smooth and fast
• looks professional

In short, a well designed iPhone or iPad app gives a clear picture of its working and makes the user feel on top of it.