Interoperability represents the ability of systems and organizations to work together (inter-operate). Due to technical constraints systems often impose, the essence of interoperability in health-care has become, in essence, the need for easy, reliable exchange of information between these systems.
Although well-intended, the meaningful use program with its billions of spent dollars has further spurred the development of multiple competing health records that are by nature proprietary. Enterprise systems, that provide the backbone of huge hospital systems are reluctant to interface with practice specific specialty "boutique" programs. Why should these enterprise systems enable integration with boutique programs, when monopolizing their implantation over a hospital system is immensely lucrative for a vendor? However, the reality is the boutique systems are often focused on end-user acceptance and walk-up usability, while enterprise systems--by their very nature, and especially by their hybrid, unfocused character--are unable to satisfy all end-users, if any at all.
How hard is it to interface through the HL7 standards developed for meaningful use and HIE? According to most experts, accepting these standards takes the willingness of the participants involved. For business reasons, many large organizations do not support standardization, and so interoperability--a functionality fully dependent on standardization-- is not an easy task. Even if a hospital may be willing to accept the interoperability challenge, the cost of implementation is often artificially elevated by the proprietary vendors, making it prohibitive.
A related factor, referred to above with the term "hybrid", is the fantasy of a one size fits all product. Hospital IT departments love hybrids because they theoretically means less hassle. Their focus is less on the end-user than on avoiding potential problems for the IT department.
Hospitals, CEOs, private practitioners, politicians, and all health-care advocates should encourage their EHR vendors to allow the concept of interoperability to be enacted upon and enhanced. You can find more information on the subject at the Center for Medical Interoperability.