Only 1/3 of the Senate was in play this election cycle. Only 1/3 of the senate is ever in play. Considering the dynamics of Senate elections (they're very safe for incumbents), the number of seats that turned over was huge.

Funny how the "throw the bums out" tradition hit the Republicans unequally two cycles in a row. Even funnier considering the approval rating for Congress was crap when this happened last November.