Even though I like it, I find the DSL too verbose. A slimmer DSL would be great. Generating the spec from existing code worked sometimes, but it coupled the API design process to the implementation code. I'm not a fan of that.
I think OpenAPI's appeal is in tooling: provide a spec, then you get documentation, API consoles, mock servers, etc. for free. If it's for documentation only, people will continue using Postman. Postman already provides mock servers and consoles, so there you have it. It'll take more tools built on OpenAPI to make it very appealing.