Democrat Chris Murphy winning a Senate seat in Connecticut isn’t a big surprise and won’t alter the partisan composition of the Senate, but it does change the ideological complexion of the body. Lieberman is a very moderate Democrat (46th most liberal member of the 112th Senate, between Pryor and Baucus) whereas Murphy is a pretty liberal Democrat.
You can see this on a few key economic issues. Lieberman is a huge fan of the Simpson-Bowles plan to the point of opposing Barack Obama’s tax proposals. He’s also an ardent free trader. Murphy has nothing to say about Simpson-Bowles and wants to partially dedicate the money from repealing the Bush tax cuts “to help pay for college and job training for the middle class.” He’s also a major proponent of Buy America laws with much more of an industrial policy orientation than Lieberman.