The interface is the gateway to which humans interact with machines and systems. As a gateway, there are many aspects of it's design and operation that can render it Good or Bad.
Since a website may garner a vast and wide range of people from all over the world; the number one goal is to come up with a solution that accounts for the different types of people that may use the interface: Beginner, Intermediate, Advanced, Old, Young, and most important of all: Learning Capability and Speed.
When looking at design from a human interaction point a view, we tend to study the human eye and the very broad range of it's ability to translate information.
The practice in determining the levels of importance in design decisions is called User Experience Design, it may encompass what follows:
Organization and Hierarchy of Information/Data
Relation of Information/Data
Progression through the System (which information is carried and what is not)
Amount of Interaction Required (Mouse clicks, and Keyboard typing)
Responsiveness (Time from input to response)
Tactile tools (What is always accessible throughout the process/system)
The altitude in which each one of these aspects is tackled can effect the outcome greatly. Doing a Bad job can translate to: frustration, lack of engagement, and even physical headaches.
Each human should be able to know what to do, how to do it, and where to go next within 7 seconds. As more and more people gain experience in working with software from a young age; that number is rapidly decreasing.