After all, we have a "karma score" here on HN and we think it's great... in part, presumably, because it's opt-in and it doesn't follow you around.

Is there a middle ground? As in, an identity service that offers this kind of "trustworthiness score" across services but it's opt-in and you can have as many "identities" as you'd like. You could use a high-trust real-name identity with certain online accounts like banks but also have the ability to use throw-away identities for anonymous browsing and commenting? If such an identity protocol included some kind of cryptographic chain of proof as a way to validate the trustworthiness, it could be quite useful.