A great user interface follows human interface design principles that are based on the way people—users—think and work, not on the capabilities of the device. A user interface that is unattractive, convoluted, or illogical can make even a great application seem like a chore to use. But a beautiful, intuitive, compelling user interface enhances an application’s functionality and inspires a positive emotional attachment in users.IOS Developer's Library
I have included a brief summary of guidelines for the Human Interface Principles, more detailed ones can be found at the IOS Developer's Library website
Consistency is where the interface takes into consideration the user's previous experience. In the case of web design, users expect the shopping cart to be in the upper right hand side of the interface. They also expect links within the copy to be underlined. Is the meanng of icons consistent not just within your site but with possible previous user experience. The same functions needs to behave the same throughout the application. We need to continue to use the same convensions that are already understood by our users.
All actions need to be initated by the user. A course of action should be suggested not automatically happen. They should be able to stop an operation at any time. The user feels like they have more control when the operations and controls are familar.