My take on Senator Joe Lieberman and his retirement is up at Greg's place.
For the most part, though, while Lieberman often engaged in arguments that reinforced Republican frames, his voting record reveals a fairly reliable Democrat who ultimately supported a domestic agenda most liberals would approve of, from health-care reform to cap-and-trade to the DREAM Act -- and, as Greg reported on this blog, he played an essential role in the repeal of don't ask don't tell. If you were a Republican who thought McCain's one-time potential vice presidential nominee would obstruct the Democratic agenda, you should be more disappointed than a Democrat who feared the same.
That said, Connecticut can field a more liberal senator than Joe Lieberman. So while Lieberman's ability to frustrate liberals on domestic matters was more style than substance, I can't think of too many reasons liberals would be disappointed to see him go. I wouldn't label him a "Democratic hero," but as far as Democratic partisans are concerned, he's certainly no Voldemort either.
Basically if you're a liberal, I think there are a lot of substantive reasons to dislike Lieberman and be happy he's gone. But if you're a partisan Democrat, you're probably pretty happy with his record in the 111th Congress.
Senator John McCain has suggested Lieberman succeed Robert Gates as defense secretary, but that seems like a poor suggestion. Aside from McCain's rather odd decision to support for SecDef the man who was most responsible for repealing DADT, which McCain suggested against all empirical evidence would cause serious harm to the military, Lieberman is decidedly to the right of the administration on a number of defense and national security issues. It would be a little odd to see Lieberman, who opposed civilian trials for terrorists and seems eager to push the U.S. into a military confrontation with Iran, serve as SecDef in an administration that nominally wants to see terrorists tried in civilian court and is pursuing a multilateral strategy as a way of avoiding a potentially devastating and counterproductive military confrontation over Iran's nuclear program.
Given this administration's record of caving on national security issues and the possibility that Lieberman's hawkishness might make him an exception to the unstated law that Democrats can't serve as secretaries of defense I could almost see it happening. Almost. But there's a difference between a reliable Democratic vote on domestic issues and someone who shares the administration's views on foreign policy and national security. Lieberman is the former, but not the latter, which would make him a poor candidate for SecDef.