Digg is facing some issues with it's new launch of V4. The core of this issue is use of Cassandra in the new version instead of MySQL which was running the site in V3. As a result the VP of engineering has to pay with his job!
As with all new, shinny "world domination" technologies, Cassandra is too immature to be used in the wild. I am not saying that Cassandra is a bad project but that it needs time to mature. World needs NoSQL databases but you cannot just shift a multi million hits website's core system from a stable, trusted, tested workhorse to a new, talented yet unproven and expect everything to just work.