Enjoying the discussion. The argument that HTTP status codes are "good enough" is one perspective. While I don't buy into everything in the "spec" proposed here, I think having application specific errors that make more sense is reasonable.

In response to there being no method of feedback, looks like the mailing list is linked up now: http://lists.omniti.com/mailman/listinfo/jsend-users

While I might no agree with everything presented in the project, I do agree that it is somewhat arbitrary (and certainly complex) mapping HTTP status codes into every JSON API.