Ask HN: How do you scale a community while retaining quality?

Let's say you have a small size site with good users. You want to grow the site but not at the expense in the quality of users.

You want to add features which will make the site more interesting (like Reddit's article voting system) but you definitely do not want the site to turn into another Reddit.

You also don't want to do stuff like create a membership committee and a bunch of red tape to join the site.

What options are out there?