Add an incentive mechanism to motivate runn a server, and hey it's a blockchain. But those have no practical application so it must not be a blockchain..
replies(4):
http://www.aaronsw.com/weblog/squarezooko
Ben Laurie read this post by Aaron Swartz while thinking about how a certificate transparency mechanism could work. (I think Peter Eckersley may have told him about it!) The existence proof confirmed that we sort of knew how to make useful append-only data structures with anonymous writers.
CT dropped the incentive mechanism and the distributed log updates in favor of more centralized log operation, federated logging, and out-of-band audits of identified log operators' behavior. This mostly means that CT lacks the censorship resistance of a blockchain. It also means that someone has to directly pay to operate it, without recouping the expenses of maintaining the log via block rewards. And browser developers have to manually confirm logs' availability properties in order to decide which logs to trust (with -- returning to the censorship resistance property -- no theoretical guarantee that there will always be suitable logs available in the future).
This has worked really well so far, but everyone is clear on the trade-offs, I think.