EMRs and all related systems (PACS/DICOM in radiology which is what I know best, Lab Info Systems, etc.) all interoperate by open (but usually fairly stupid) standards like HL7. They are basically the worst of OSI, ASN.1, etc. all rolled into one. But, they're standards, and there is a huge amount of workflow deployed around them -- $5-10mm imaging devices that speak these protocols, entire labs, huge numbers of doctors trained around them, etc.

There's no way a records presentation system would be useful if it didn't take standards like HL7, ICD9 codes, etc. on the input. There has been a huge effort over the past few years to get small practices onto EHR/EMR (for a variety of reasons...patient care quality and billing/admin cost savings), so anything which didn't work with those standards would be a huge step backward on those efforts as well.

The nice thing is most of these ugly protocols have decent libraries available (some open source, some commercial), so you can build a HIS/EHR/EMR/PACS/LIS/etc. with general software engineering talent and a limited amount of specialty knowledge, but you really do need some experience in the clinical environment (at least as a tech or doing tech support), I think, to build a credible overall system. And to actuall sell larger systems, an MD on the team is really helpful (at least for PACS, a rad or ortho).

That said, being able to 'pretty print' a medical record (on paper, or maybe some kind of interactive format) for patient use or portability outside the system doesn't require a huge amount of clinical knowledge, but it's not enough to replace an EHR.