4chan, specifically /b/, attracts a bunch of juvenile idiots because it's targetted at juvenile idiots. No matter what the owners / founders originally had planned, that's how it turned out.

Also, I think 4-ch is actually quite well off concerning the good post / stupid post ratio. More posters means more quality posts, sure, but it also means more crap posts. The relative amount will stay about the same.

The only thing that's worrying me a bit right now is how DQN is bleeding over into general. That which is DQN, should stay in DQN.