Looking at Alexa's numbers for a rough comparison of pageviews it looks like Digg gets about 10x as many as SO:

http://www.alexa.com/siteinfo/stackoverflow.com#trafficstats

So while SO runs on a minimal amount of closet-room hardware efficiently and without incident, Digg has heroics and clusters and scale-outs and all sorts of drama to handle just 10x the load? I'm not sure what angle you were coming from, but that makes Digg look like clowns.

Not saying that Digg aren't clowns but the traffic balance makes a huge difference. It's not implausible that Digg has an enormously bigger fraction of writes to reads than SO does which changes the scaling dynamics considerably.

Edit: To expand I'd imagine SO serves a lot of static "how do I" google hits for logged-out users which is about as easy to serve as it gets. Digg puts a higher emphasis on the logged in experience and commenting which reduces the ability to cache.