In most commenting platforms, comments last forever, and anyone can write a reply to any comment. This tends to lead to nasty arguments, because you end up arguing in public trying to defend dumb things you said earlier.
In Hubcard, you cannot reply to a reply. Instead, if someone disagrees with something you said, you edit your original comment to be less wrong, or to be more robust against that criticism.
When you edit a comment, any replies are hidden until they are themselves edited in response to the edits you made.
The end result is a space where you see the best version of an argument and the best response to that argument.