The hardest part about API design is picking the appropriate primitives in the first place. Use cases are relatively easy--somebody has a problem, come up with a solution--but any solution can be expressed in a myriad of different ways. The way that the API designer expresses that solution to the users of the API will impact the way code is written.

Users generally won't understand the implementation of the API, so instead they build a model of how they think the entire system operates. In the process, they make assumptions about performance, side effects, etc. that may be totally incorrect. Good APIs anticipate these assumptions and make them relatively easy to predict with some level of experience, either from elsewhere in the API or from similar APIs.