Jason Bay @ Gamasutra: "A designer's most important job is to craft unique and interesting challenges for the player. Too often though, we discover that players are unexpectedly challenged by aspects that should be easy or even invisible during gameplay.
These might be the workings of the user interface, or an in-world interaction that should be a snap to figure out but somehow just isn't. When these things get in the way of normal progress, players become frustrated and can stop playing your game altogether.
As developers, we try to discover these problems early and rework the objects in question until they approach a better semblance of usability. But the journey from "this sucks" to "hell yeah!" can take a lot of work and iteration and the goal is often reached more through intuition than on established process."