Slashdot implemented something in something similar long time ago (I guess they still do it) and I don't know why nobody ripped that off. To start moderating your account need pass a certain threshold of "oldness".
I think the moderation is even more important because when people start modding down valid coments or modding up stupidity that's what make me want to leave. That's when I got annoyed with Digg +-2 years ago and Reddit +-1 year ago.
I've been wondering that too. Slashdot has gone through all of the problems that face digg, reddit, etc. And they have found some pretty good solutions through trial and error. They are even well documented here: http://slashdot.org/faq/com-mod.shtml
Generally I am amazed that moderation systems are not better than they are, especially since the stakes seem to be so high in this niche right now.
I think the moderation is even more important because when people start modding down valid coments or modding up stupidity that's what make me want to leave. That's when I got annoyed with Digg +-2 years ago and Reddit +-1 year ago.