I think it's great that they are attempting to do something about the downward trend of posts/comments on HN but I'm not convinced that this is the solution though. There are so many instances where I can see this having a bad impact - especially in threads that aren't as popular, threads that are a bit older, etc. I'm not sure how HN karma works either. Is 1000 a lot? I have 24 but I've probably only commented and posted 2 things. If 10% of the active users have 1000+ karma, this is going to be disaster. If 90% have 1000+ karma, it might work better. Will those with 1000+ karma see if a pending comment is pending or not? The inherent "responsibility" that users may feel when upvoting a pending comment might have an interesting effect on the way they vote.
I expect this mechanism to have a chilling effect on HN. I have long been a lurker there. Recently I have been using it more frequently, thinking that it stands as a nice alternative to reddit (which I have followed since its birth), but now I wonder if I will ever be able to participate in a real-time discussion there. If I cannot participate in such discussions, how can my karma reach 1000?