This week's Big Thing Question seems to be why Democrats allegedly don't play "hardball" as well as Republicans do. I'm not really sure whether that's true or not, but I do agree with Kevin Drum that Republicans are probably a bit better at believing their own press releases; with Drum and Matt Yglesias that the GOP partisan press gives some tactical advantages to Republicans (although I'm not really convinced that those advantages play out to anything more than winning unimportant spin wars); and with Jamelle Bouie that Democratic unanimity is harder to achieve because Democrats are simply more diverse: what makes Democrats the larger of the two parties is the same thing that makes it harder for them to stay united.
Before I submit my own addition to this, I should point out: the 111th Congress has been one of the most productive Congresses ever. That doesn't prove that Democrats are really better at legislating that the current conventional wisdom would have it, and it certainly doesn't mean that disgruntled Democrats should just shut up and accept that their pols are brilliant. Not at all. But it is worth keeping in mind that the context of all this is "not enough" rather than failure. That said...
I think Bouie is on the right track here, but I think even more to the point is that most Democratic constituency groups have real policy demands, and that they're very eager to have those demands fulfilled. My sense is that a lot of Republican constituency groups have more symbolic demands.
Therefore, at the end of the day, a lot of Republican constituency groups are willing to go along with an all-or-nothing strategy on most issues, while Democratic constituency groups are perfectly willing to bargain for as much as they can get. Look: if you want universal health care, you are probably willing to settle for moving from 80% coverage to 95% coverage (or whatever the actual numbers are). If you believe that government involvement in health care is unconstitutional, or immoral, or whatever, then there's not much to bargain over.
A lot of liberal commenters noted, and I think were in some cases surprised, that moderate Republicans weren't willing to cut a deal on ACA (or the banking bill, or climate), since given the political situation those Republicans could basically write their own ticket. I think the answer may be that those moderate Republicans just didn't have important constituency groups with specific, discreet policy demands in that area. Not just in health care, but on quite a few issues that Democrats consider critical -- because Democratic constituency groups consider those policy areas critical.
In some ways, that's what makes John McCain the perfect Republican -- because he simply doesn't care about policy at all, except in personal or symbolic ways.
Yes, I should point out, it's a generalization that doesn't always hold, and probably an exaggeration. Nevertheless, I think it accounts for some key differences. In lots of policy areas, Republicans simply don't care very much what happens. And while that has a lot of limitations, it also can at times give them strong bargaining power.