Documentation and ease of use. The API should function at the highest level possible. I should be able to call the service with concise and powerful statements. The beauty of an API is the ability to quickly build further upon something that has already been done. If I have to go ten rounds with it to get it to work, you've missed the boat.

In that same vein the documentation should be clear and to the point. See Amazon Flexible Payments as an example of what NOT to do. Ambiguous docs with tons of circular linking etc. made me want to kill myself and did end up killing my project. Just my .02.