I'm not sure I'd agree with this. Digg seems to be more sophisticated technology wise than pretty much any competing site. Besides some basic scalability issues and a few bugs when launching a new features, all these sites don't require much in terms of engineering. It's much more a question of content programming and community building. Reddit and Hacker News may look successful because of an engineering culture, but I'd wager that's more about having a strong identity than the site's underlying technology.