[bonoboTP]

I think it's the people and the nature of the "hacker" topics. You need a certain amount of attention span to read plain text comments, enjoy a front page with no images, you need patience to learn programming and tech stuff. You need some explicit thinking and less knee jerk reactions. There is some actual object level to discuss, some objective technical things, not only who stands for what and who likes who political games.

Of course this could be reproduced in other similar communities as well.