I'm more concerned about the soft features of an API. That is:
- It just works
I could right a book on how those apply or would get implemented, but suffice to say unless a user of the API can wrap their head around how to use it effectively, it's of limited use. Simplicity does not mean simplisticness, just as elegance does not mean onate. It simply means that it is just perfect for the job. The less someone has to think about how to use the API, the more they can just use it.
How these look depend on the lnaguage, purpose and target audience of the API. The very last criteria boils down to the principle of least surprise. No errors where you didn't expect them. Any reasonable interpretation of the API will get you the results you want.