In transitioning from Swagger 2 to OpenAPI 3, the author highlights the ease of migration and the practical benefits of new features in OpenAPI 3. Among these features, the ability to mark properties or operations as deprecated simplifies the process of acknowledging and managing past mistakes, as visually supported by Redoc. The write-only attribute offers a straightforward solution for securely handling sensitive information, crucial for compliance scenarios like PCI DSS. Additionally, the nullable attribute allows for flexible data handling by accepting null values, previously managed through vendor extensions in Swagger 2. Despite various features, these three—deprecation, write-only properties, and nullable attributes—are emphasized as particularly beneficial for enhancing API definitions.