Why no reply envelope icon in the corner?
Please, don't get me wrong, I love the simplicity of HN. But I feel like many threads get abandoned by their parents because there is no indicator to let them know someone has engaged in conversation. A simple reddit-style "orangered" would fix that. Nothing more - just that.
This is asked often, and answered often. Lack of reply notifications are a feature. It drives community-driven discussion, as opposed to 2 people arguing with each other. Say what you have to say and let it be - other people will reply if they have something to say. Don't worry about getting in the last word, as it is perfectly fine for the originator of a thread to not participate in downstream comments.
Unnecessary. There is a "threads" link in the header. It shows all of your recent comments, and their replies.