Once upon a time, there was Chips and Dips, and rumour has it it was quite good. Then came Slashdot, and it too was good. It was so good that many people enjoyed reading it, and they told their friends, who told their friends. The comment system increased the number of visitors too, and very quickly Slashdot became very very popular. This had several side effects.
The most obvious affect was that the server couldn't cope with the load. The less obvious affect was in the quality of the comments, which seemed to deteriorate as more users joined in. Moderation was added, as was Meta-moderation. Which helped quite a bit. But neither was a perfect solution, and in some ways caused their own problems.
Whatever the niggles with Moderation, the wider problem still remained, that tens of thousands of people were all trying to be part of a small community, and get their point of view heard at the same time. The discussions slowly got less coherent and less useful. And now many have given up on Slashdot, looking elsewhere for something better and more informative.
The rule "All good things come to an end" probably applies to websites as much as anything else. Slashdot demonstrates just what happens when a small community's size increases massively. It's not that there was anything wrong with Slashdot, it just happened. It'll happen to other sites too.
Of course, those who share this view could be being pessimistic, and Slashdot (or other sites) might carry on riding that thin line of "just useful enough" that keeps the crowds coming. Time will tell.