It was more a comment about how considerations such as this make collaborative recommendation systems precisely less useful. Regarding rec.systems that move bey

It was more a comment about how considerations such as this make collaborative recommendation systems precisely less useful. Regarding rec.systems that move beyond naive rating/like intersections between users (such as e.g. Jaccard similarity index), I think there could be interesting algorithms developed that make use of assigned/curated item tags together with collaborative filtering/recommendations (e.g. something that last.fm has considered (or has been considering) - there's an entire field of research for just music recommendation systems.)