I had a bad feeling about implementing the official extensions, or really any extensions at all to the spec, since extensions may completely break the base specification.

In a comment regarding content negotiation, Roy Fielding writes:

it is a waste of time to negotiate on the basis of obscure parameters while using a meaningless media type like vnd.api+json. Define separate media types (preferably with meaning) if you think the differences will matter to a client. You will note that the examples in the spec are artificial, since there isn't any real-world need to negotiate on parameter names.

"meaningless media type like vnd.api+json"... ouch ;)