For me, success was less about creating the FHIR resource, or querying a FHIR server for a particular piece of data, and much more about solving a particular workflow problem. It really wasn't about FHIR at all. I spent so much more time thinking about how I wanted to address this problem, and designing the service to work the way I wanted it to than I ever did worrying about the details of FHIR. And that to me is the biggest sign that FHIR will be successful. It gets our of your (or my) way, and lets you (or me) focus on the real (or externally imposed) problems.
Grahame Grieve, Ewout Kramer, Lloyd McKennzie, Josh Mandel, John Moehrke and many others have done a tremendous amount of work on the FHIR specification. I look forward to resolving the ballot comments on this specification [something that I rarely say]. I'm getting quite impatient to use it, and thrilled that it has become the basis for the Blue Button Plus REST API Pilot efforts.