There have been some great responses that cover the gist of it, but to add a couple more mundane specifics: - Build up the thinnest abstraction that lets you e

There have been some great responses that cover the gist of it, but to add a couple more mundane specifics: - Build up the thinnest abstraction that lets you express a solution in the problem domain. - Allow the API user to express their intent, and not be focused on mechanics they don't need to understand to get the job done. - Focus on getting the interfaces right before the implementation is perfect. - A clean interface will allow you to make drastic changes to the implementation without the API consumer becoming aware.