As communities grow larger, and become Slashdot-size, the amount of traffic begins actually begins to stifle the community. Good ideas are lost in the flurry, and the best stuff goes often unnoticed. The problem of just how to solve this problem is still being worked out on thousands of sites with slightly different methods.
Clay Shirky makes a living writing about these kinds of issues, and his piece "A Group Is It's Own Worst Enemy" is the best I've read on the subject. Borrowing liberally from his thoughts, I put together a piece about the Dailykos website, which is facing exactly this problem.
If you're interested in such things, check it out.