But then miners get cheap reputation.  Miners can collude to upvote each other.  You also need negative ratings (aka antizaps) for an optimal reputation system.  Opentimestamps have the issue of who stores the data.  Reptuation has to be able to evolve, rather than be a static timestamp.  

This popped up in the last few days, which may have some promise.

https://github.com/cromatikap/nostr-reputation-authority