There is almost always a main idea to any given topic (it starts and ends by saying something specific).

> And how do you even decide what's on topic and what's not?

Good point. But in general, one can sense an overall picture emerge out of a comment stream. As you scroll and read comments, you can tell when the stream provides value or not. My point here is, that when people do not stay consistent to the original topic, the value of the comment stream goes down. And here, I'm not talking about comments being totally out of context of any given original topic. Even a slight divergence leads to unnecessary path down the road.