There is a spanish Digg-like site, Menéame [1], which offers this same option. I think it works pretty well as it keeps the site clean of news which don't obey the rules. Despite that, people will still reply explaining why they flagged the comment (and I think this is the correct way to behave: you should explain why the comment is wrong and how to improve it in order to improve the community)

[1] http://www.meneame.net