Getting a post popular on Digg is a complicated story. You mostly have to bribe or play tricks to get there. Getting there by simply submitting a story and being active in the community is not very common. Try other social bookmarks to get traffic, Digg looks like a total waste of time to me at least. Getting tonnes of traffic from SU is much easier but again as everyone else says, the traffic do not convert well.
Honestly I find this one a tough one. I wish it was about writing good work but do not believe so. I believe multiple Digg pieces are based solely on topic and very few are actually read for content. I've been there 3 years and it just seems to never change. Don't know if that helped.