I strongly believe that any non-PoW-based method to filter spam, which must rely on subjectivity and "rough social consensus," is doomed to fail. Worse, it has a centralizing effect, mirroring the dynamics of Proof-of-Stake. This "social consensus" filtering is just PoS by another name. We've seen this cat-and-mouse movie too many times, especially during the internet's evolution (see: email, SMS, DNS, social media).