What's missing is fully functional, advertised implementations of the Service Provider, that work with LDAP and ideally older Kerberos backends. What's missing is the native, built-in interface. What's also missing is a SLA... big players can't adopt it unless Mozilla says they are going to support it. What's missing in Persona is a promise to have the code security audited and get it passed federal agencies. What's missing is some grass-roots effort to get it into universities.
What they list as failures aren’t exactly. So, there are some UI issues.. but those aren’t the problem. The Persona service was meant as transitional, till native implementations appeared. It's not going to adopt itself, they need a "Sales Engineer" full time working on adoption. Hell, they could even charge people for that sales engineer and many would pay for it. The alternative to Persona is Shibboleth/Kerberos, and Persona kicks it.