So far, the lame-duck session has managed to pass an $850 billion tax-cuts-and-stimulus deal, the repeal of DADT, the Defense Authorization bill, a continuing resolution to keep funding the federal government, the START treaty, the food-safety bill, and probably a few more pieces of legislation I'm forgetting. This is vastly more than anyone expected, and even if I'm disappointed by the failure of the omnibus spending bill (for reasons explained here) and the DREAM act, I can see why Sen. Lindsey Graham summed up the session by saying, "When it's all going to be said and done, Harry Reid has eaten our lunch."
But it wasn't really Harry Reid who ate their lunch (and how much better would that quote have been if Graham had said, "Harry Reid drank our milkshake"?). It was the Republicans. DADT repeal passed because Sens. Susan Collins, Lisa Murkowski and Scott Brown voted with the Democrats. The tax deal went through because a host of Republicans voted with the Democrats. Same for START, the food- safety bill and the DoD authorization. If the bill helping 9/11 responders get medical benefits passes, that too will be because of Republican support.
The question is why the Republicans didn't just drag their feet and let things expire and then come back to everything in 2011, when they'll have more allies in the Senate and control of the House? As Graham said, "with a new group of Republicans coming in, we could get a better deal on almost everything."
The answer, I think, is that there are plenty of Senate Republicans who aren't too comfortable with the class of conservatives who got elected in 2010. These legislators knew they had to stick with McConnell before the election, as you can't win back the majority by handing the president lots of legislative accomplishments. But now that the election was over, the bills that had piled up were, in many cases, good bills, and if they didn't pass now, it wasn't clear that they'd be able to pass later.
The incumbent -- and the outgoing -- Republicans know that the fact that Republicans will have more power in 2011 doesn't necessarily mean that they'll use that power to pass sensible legislation. So those of them who wanted to pass sensible legislation decided to get it all done now, even if that meant handing Reid and Obama a slew of apparent victories in the lame-duck session.