Very nice to see you back here, Peppermint Pig!
The major difference is that this forum content is owned by the people that we apparently trust. There haven't been any major reasons to call them biased. (I'd be more comfortable if they were more "open source" about the data and provided periodic XML / static HTML forum dumps to proliferate information in case the central server goes down, but I certainly could run my own spidering / mirror script if I wanted to.)
The Digg content, on the other hand, is owned by a large company and is likely to be bought by an even larger company in the future. It is becoming as dominant in the social bookmarking market as Microsoft or Google are in their own markets. Not that there is anything inherently wrong with that, but the larger a for-profit company is, the more pressures they are subject to. By contributing and especially by linking to their site (which I have been very guilty of recently), aren't we just helping to create another CBS or CNN?
More importantly, we are not challenging ourselves to come up with our own grass-roots solutions for sharing and popularizing news stories. Over-dependence is always bad, especially when there is centralization.