Interesting thoughts from Melvin here.

Personally I think that, to misquote Szabo, "social verification does not scale" and that's why I came up with "RIDDLE" and then aut-ct though, to be clear, that's a set of ideas very focused on keeping privacy along with Sybil resistance, but as a side effect it does not "pollute" the main chain nor have related, nasty user costs.

The biggest negative of those ideas is that the Sybil resistance effect is negligible at small scale, it will only defend against large scale spam/DOS, but I think that's a good tradeoff against it costing the user absolutely nothing.