According to the Smartbear State of API 2019 Report, the 3 most important characteristics of an API are:
- Ease of Use
- Accurate and detailed documentation.
This response was from Smartbear’s survey of over 3,000 developers, architects, QA professionals, operations engineers, and product leaders from more over 16 different industries. The survey was run between November and December 2018. Here are the full graphed results for the question posed.