I remember a few years ago some analyst claimed that without a working PKI Internet commerce would never happen. Billions of dollars later, I think the industry has pretty loudly voted with its feet. Rik Farrow replied by saying that it will take an organization with big bucks and a requirement for PKI which will show a substantial return before we will see any large-scale deployments. The cost of implementing PKI, plus the complexity of ongoing management and the non-technical considerations already raised certainly do create barriers. The token environment works well, but we'd need an international standard that can be adopted by all government, commercial and consumer users, plus a means to add on the encryption and still meet the various legal and business requirements for privacy, universality, etc.
Cost is the primary barrier.