His continued support of the Iraq War and attacks on the patriotism of its opponents are the main reasons for opposing Joe Lieberman, but his history of attacks on fellow Democrats and liberal ideals is much longer. There was his support of DOMA in 1996, his scolding of Clinton during the impeachment hearings in 1998, his flirtation with private accounts during the early days of the Social Security fight, and his cloture vote on Alito. While his overall voting record isn't bad, it's hard to see Ned Lamont as anything but an upgrade, especially as far as winning media battles is concerned.
What's really exciting about this race is the message it'll send to safe-state Democrats who may occasionally be driven to Liebermania by the siren song of the anti-partisan media: bashing your party for personal gain is unacceptable, and Democrats in safe seats are expected to do their part in moving the country leftward. Betrayals of the party and liberal ideals, whether for reasons pragmatic or psychological, may come with the consequence of a primary defeat.