A minor part of the equation is probably some sliver of good will the president garnered from GOP moderates with the tax cut compromise, which gave Republicans most of what they wanted, by some measures, virtually all of what they wanted.
Another critical part of the equation is that the very size of the Democrats' majority in some ways paradoxically worked against them. The shift in power -- both taking muscle from the Republicans and giving more to a still fairly small GOP Tea Party subcaucus -- has given the fabled 'GOP moderates' a bit more room for maneuver.
But as I've watched this unfold I've had an unmistakable sense of that thing that is always supposed to be an illusion in politics: momentum. Each win seemed to build on the previous one, with the unexpected 9th inning victory for DADT repeal somehow bringing the START Treaty and the 9/11 bill along with it.
I haven't been able to put all the pieces together yet. But my first take is that each time a bill was allowed to get an actual vote it undermined the rationale for denying a vote to the next thing. The fact that DADT Repeal got 65 vote (two more than the filibuster-breaking vote) was an early tell. START also passed with votes to spare. One of the most pernicious things about the modern filibuster is that the filibusterers rarely even have to cast a vote. It's all passive and thus hidden from all but fairly close observers of the process. And those folks mainly already have strong opinions. All of which means there's little cost to obstruction.