The second point is great one. Occasionally I have to stop reading HN and leave the tab open for a day because I want to read the conversation in full instead of rereading again later trying to figure out which comments are new.
I had the exact same problem. There are a lot of interesting discussions, but its way easier to distinguish the interesting from the useless discussions when they're done instead of ongoing.
I find that I read more carefully and focused when the discussion are already crystallized, instead of just browsing the latest discussions for some dopamine.