And there is an aspect to that ability that I know people don't like.
The ability to collapse means you feel less bad about somewhat-off-topic conversation, which in a way can create worse discussion for the actual topic.
But at the same time some of my favorite HN threads have been offshoots of the submission's topic.
I don't remember wanting for collapsable threads and such four or five years ago, but now I can't imagine sifting through all the junk without them.