This is a silly idea for many reasons and I won't get into all them.
Turning HN into an invite-only site will radically change its nature and reduce the quality of posts and variety of opinions found here.
Apart from indulging in some serious groupthink, you'll keep out new or existing hackers that haven't found this site yet.
The site keeps growing and so the nature of the site changes. Instead of using draconian measures in a doomed attempt to recapture the 'good old days', make changes to the community system (karma points, modding, etc).
Turning HN into an invite-only site will radically change its nature and reduce the quality of posts and variety of opinions found here.
Apart from indulging in some serious groupthink, you'll keep out new or existing hackers that haven't found this site yet.
The site keeps growing and so the nature of the site changes. Instead of using draconian measures in a doomed attempt to recapture the 'good old days', make changes to the community system (karma points, modding, etc).