[fcanas]

I'll add two points that I think are more important than any that have been mentioned so far.

1) A good API is hard to misuse. – Documentation is great, but what's even better is when the naming conventions and responses from the API are so well named and their purposes are so obvious, that it's hard to misuse.

2) A good API is helpful and informative when you've misused it. – Sent in an invalid key? The response should say so, and not just return an anonymous error. Is your request missing a required header field? A required parameter? An incorrect type for a parameter? Unused parameters? Yes, good documentation can help with these issues. But one of my biggest problems with documentation is that you can't look up what you don't know you need to look up (especially when there aren't any good tutorials).