Also, everybody can 'flag' articles that are not suitable.
What was your favourite social news site before ?
And I flag a lot of articles, but very few of those get banned, same principle as with submitting. If a few people flag and a lot vote up, it stays. And "few" and "a lot" are relative terms here. They'll change with time.
I've seen narrow and deep focused discussions be replaced with wide and shallow discussions on many sites, but in rough order: Slashdot/Digg/Reddit/programming-reddit/science-reddit and I know I'm forgetting a few that came and went fast.
The process is known as "the September that never ends".
Slashdot is a bit better now days though the rate of submission is slow.
When PGs essays first led people away from Slashdot to Reddit, there was a noticable drop in the quality of Slashdot. And early on Reddit was a like beautiful jewel of social news sites.