Story quality is directly linked to user growth. When you have a small niche site with "niche-expert" users, you get a high interest and visible actions; as the site grows, individual actions matter less, the niche is expanded into those that may only have a casual interest in the topic, and the forum inevitability changes to meet the needs of the average user - which is not the same type of user that previously existed. As the more windows are broken, the more "decay" is introduced into the forum. But the site is doing exactly what it's supposed to do - mirror the average demographic. It's that demographic that changes.
You simply cannot grow a website like this and maintain a niche focus. So if you choose to welcome more and more people, this is the end result. The sites that have been successful at maintaining focus control either the story submission (/.) or the entry of users (MeFi).
Sites like HN and reddit need to limit growth the same way a small startup shouldn't focus on growth per se if they want to retain that particular environment.
How to do this successfully is another problem altogether.