Currently, the REST interface gives very vague error messages when field definitions etc do not validate. The error messages doesn't for instance say what field definition did not validate, just that "some" field did not validate
As and example read issue EZP-20606, and how the current implementation handles this in my comment from "02/May/13"

Better validation feedback is a must when creating GUI based on REST ( not only when creating content types, but also when creating content objects etc... )