The Web API Checklist 43 Things To Think About When Designing, Testing, and Releasing your API - Mathieu Fenniak
mathieu.fenniak.net/the-api-checklist
What Jamie wrote
This is a good list.
I dont want you to build an API and only know about your unknown unknowns when they bite you in the ass. So, heres a list of a bunch of things, both obvious and subtle, that can easily be missed when designing, testing, implementing, and releasing a Web API.