Sybil is on my mind.

I plan a register of persons to prevent fake multiple identities. Assuming a core number of trusted users, every user would show up in person at a certain number of users in his area (partially to be selected by random choice, to prevent systematic collusion). The user would present some official documents and then get a uid based on a standardized id scheme, eg a hash on "Firstname Middleinitial Lastname-When-Born Birthplace Birthdate". To be able to act anonymously, the user would first logon using this id and then use this to generate secondary anonymous credentials for login. These will be produced in a manner that there will only be one credential for every user per time unit. Thus, the user is anonymous AND the risc of a large number of fake users is small. It is kind-of a decentral implementation of a PKI, similar to the web-of-trust in PGP (but preventing collusion attacks).

Having prevented a Sybil attack by that mechanism, a trust based system is now possible as next level or user management.

The disadvantage, of course, is the requirement to establish identity first, which is rather tedious.

I can handle this disadvantage, since users who did not establish identity may use the system but will not have additional access rights and will not have rights in the trust layer. Of course, this is a not feasible for Bitcoin, because there is essentially just a single functionality (no different access right levels and no trust) and users would be very frustrated, if they had to do all kinds of registration before receiving or making payments.