The rise of the System D economy and trading between anonymous parties will promote a shift towards a reputation-based economy to facilitate commerce, so I think it's important to create a reliable web of trust system to support it. We already have the bitcoin-otc web of trust, which works perfectly fine now but has the problem of being centralized, and so it can be manipulated. Just like with currencies and dns, a decentralized web of trust would be preferable.

So basically the idea is to just embed a WoT system into a blockchain (with bitcoin-otc rules or similar) using a special transaction that says "address X gives a rating of +1 to address Y". Also it should be possible to transfer ones rating from one address to another to be able to launder it in case that the identity of the owner is compromised.