1. HN doesn't have an issue with comment quality.
2. HN should be concerned about growing the community, and increasing comments. A lot of discussions already suffer due to a lack of activity. This is going to do the opposite, it's going to decrease comments.
3. We live in an instant world. Pending comments is a step backwards for user experience.
4. Occasionally I see a topic with 10 comments, the majority of which were written back and forth within the first hour of the topic. You're going to kill these discussions.
5. Manually moderating topics doesn't work for communities like HN. It works on a blog, where your article from last year gets another two or three comments every month, half of which are spam.
6. You're creating unnecessary work for members in the community. People come here to enjoy the community, not to moderate.
7. It's a poor method of moderation. You can have 99 users decide not to endorse a comment, then one person decides to click the endorse button. 99% against, and yet it's approved.
8. I'll have to question every comment I write, and avoid spending time on any detailed responses, because they might never leave the pending stage.