I'm not talking about the events themselves or how significant they are—I'm only talking about HN comment threads.

Often, a sequence of related stories (S1, S2, ..., Sn) produces threads that are more or less the same as their predecessors, rather than focusing on the specific new information introduced by any Si. This particularly happens when the topic is a major and divisive one, like the current one.

What happens in this cases is that people tend to post their generic views about $Person or $Topic, often in vehement terms and without much curiosity about the specific details of what's happening. In this way we get threads that don't differ very much from one discussion to the next. That's what I mean by "interchangeable".