Yep, graph structure of topics enables users to wander around topic space. Good for less directed, more exploratory searching.

Another graph that is useful is the graph of people -> topic clusters. See https://twitterverse.net/ . Such a graph can help rank content from people deeply invested in a particular topic, and its hard to fake because they would presumably have to trick all their peers about their expertise

We'll have our own Show HN soon but it's great to see similar ideas bouncing around. Would love to connect over email to learn more about your thoughts.