I like Mitchell's Vouch idea. At the end of the day, it's all about trust. Anything else is an abstraction attempting to replicate some spectrum of trust.
Adding this type of rep system would destroy a lot of what is so cool about the internet though. There’d probably be segregation based on rep if it’s very visible, new IDs drowning in a sea of noise. Being anonymous but with a record isn’t the same as posting for the very first time as a completely blank identity and still being given an audience. Making online comms more like real life would alleviate some problems but would also lose part of the reason they’re used in the first place. I don’t see much any other way to do it besides maybe a state-provided anonymous identity provider (though that’s risky for a number of reasons), but it’s going to be sad to see things go.