Developer Portal
The self-service surface where consumers discover APIs, read documentation, obtain credentials and test calls — the main determinant of adoption.
For an API that is a product, the portal is the product's user interface. Integration effort is dominated by how long it takes a developer to get from arriving to a first successful call, and that interval is set by the portal rather than by the API's design.
What materially shortens it: generated reference documentation from the specification, so it is always current; guides and worked examples, because reference alone does not teach the sequence of calls for a real task; self-service credentials and a sandbox, removing a human queue; interactive try-it against the sandbox; and generated client libraries.
The operational additions that reduce support load: a changelog, a status page, deprecation notices in context, and clear error documentation mapping each error type to a cause and a remedy.
For internal platform APIs the same applies with the same effect — an internal API without a portal is adopted by whoever the team can talk to personally.