A lot of the problems with decentralised reputation systems (or indeed decentralised anything systems) stem from the difficulty of establishing a decentralised identity system.

I wish that the idea of "idchains" had been explored more, but it seems it didn't get much further than a (now unlisted) video on the inventor's YouTube channel:

https://www.youtube.com/watch?v=xZC98s4paYY

The (perhaps controversial) idea is that you accept that your face is public data, and so use that as sort of the unique key in a global database of faces, or ideally, hashes of faces. A user could then carry around a digital copy of their face picture (in a mobile app, say), which other users could attest and check the hash of in a global blockchain.

There would need to be gamified incentives to do this attestation (effectively key-signing parties for non-geeks), and you could opt to have people take pictures of you each time and upload them to a decentralised file store (like IPFS). Organisations could then compete on using machine learning to generate confidence scores that a given identity really had been in use for a significant duration, vouched for by other individuals, and not a duplicate of another account somewhere else in the system.

There are various trade-offs in terms of security and usability and anonymity, but it seems like an idea that is worth exploring.