Unless Digg cannot roll out new features due to Cassandra (eg: a) Cassandra woes taking all dev team time, or b) tech limitations prevent features) it seems highly unlikely that cassandra is the reason why Digg 4 is grating with users initially.
It's a change of features/product rather than technology that is the problem.
I think the folks at Digg where prepared for the heartburn associated with the content changes. They weren't ready for some very serious issues with the implementation. Digg has definitely seen a lot of downtime over the last week or two.
Digg has had some significant downtime during this transition (Can't blame Cassandra - but can't clear it either). Issues with product features are one thing - but nothing probably got more people jumping to a different ship (reddit) then not being able to find a good lolcat when you're bored, and your favorite news aggregator is down. Hmm - I wonder if fark has seen the same uptick as reddit has.
It's a change of features/product rather than technology that is the problem.