One X-factor in the Dems' run of lame duck wins was the GOP. They didn't exactly make it easy for Dems to do stuff...but they could've made it harder. Much harder. They ceded back hours of time; they handed Dems the food-safety bill after Dems basically botched it; they wrote this high-dudgeon letter vowing to block all Dem initiatives until taxes and spending were resolved, but then basically didn't.
I think there were a few reasons for this. Part of it was that they knew Dems had more time on their hands than most people realized, and were willing to eat into it. Another was that, with the election over, there was little value added to blocking everything, particularly for moderate Republicans.
But Republicans must at some level have understood that some of these things weren't going away. DADT would've stayed on the agenda. 9/11 responders would have stayed on the agenda. DREAM will stay on the agenda. And I'm guessing they made the simple calculation that it would be easier and wiser to give Dems these victories now, rather than fight it out with them publicly next after the GOP takes over the House with a caucus that's divided over these things.
Now the issues are off the table, and that creates more space for them to set the agenda.