Most queuing systems require some type of serialization, other than the default one implemented here. I think leaving the details and requirements of serialization entirely to the queuing system will result in chaos to the point of making this API difficult to rely on in any third-party code (gems, etc.).

Ruby already has (several) built-in serialization protocols. Requiring _dump/_load at minimum does not strike me as a particularly onerous requirement. Individual queue systems can implement more sophisticated serialization for objects if desired.