There is surprisingly little written about good API design. Maybe it's because the important things to say have already been said, but I have my doubts that that is the reason.
I suspect it's partly because of the difficulty involved in measuring the quality of a given API's design. How do you separate the success resulting from the usefulness or popularity of the technology from the design of the API?
That was the first thing I though on starting to read this. How do they know they've actually got it right? Feedback from users?
Edit: sorry this was replied to smunz's comment.