Tim Fernholz offers four suggestions for how liberals could make the tax deal more appetizing. Liberals might have some leverage here to improve the deal a bit, although I doubt that there's any wiggle room at all on the estate tax -- as it is, Jim DeMint is already against the deal based on the retention of any estate tax at all.
Before I offer an alternative list, I should point out one important aspect of this: electoral politics is zero-sum, but policy is definitely not zero-sum. In electoral politics, for every gain Democrats make there has to be a GOP loss. But in policy, win-win trades are not only possible, but quite likely. So it makes sense to think of what Democrats might want that is worth more to them to get than it would cost Republicans to give (and vice versa).
What do I think Democrats should ask for?
First, I agree with Fernholz's last suggestion, the debt ceiling -- although what I'd suggest is what Ezra Klein has argued for some time now, that they should just toss it into the agreement now, rather than just get some positive public statements. I wouldn't be surprised if that couldn't get done; from what John Boehner has said I don't think he's looking forward to a debt ceiling fight with a caucus he probably can't control on that issue, and I've argued that it's quite dangerous to his position as Speaker.
Second, I think the Democrats should bargain for whatever they can get on other, unrelated legislation -- for example, allowing the food safety bill to get through whatever parliamentary fix is necessary for it to advance to the president. Ideally, that would include the three big remaining items on Barack Obama's lame duck agenda -- DADT repeal, DREAM, and New START (and yes, that's a lot of all-caps items). I'm not sure whether there's any bargaining room in there at all, but remember that for the most part Democrats wouldn't be asking for Republicans to vote yes; they're just asking for less obstruction. If, as is likely, there's no give on those, at least perhaps Democrats could bargain for the Senate to clear some lower-level, less controversial measures, with food safety probably at the front of the line.
Third, there are still quite a few uncontroversial executive branch and judicial branch nominations pending. There's no way that conservatives are going to clear liberal Goodwin Liu for the 9th Circuit as part of this deal. But there are nominees for Under Secretary of Management at Homeland Security, Under Secretary of Export Administration at Commerce, Deputy US Trade Representative, General Counsel to the Army, and many others that as far as I know just didn't happen yet, and probably won't without Republican cooperation.
The other half of the equation is about what disgruntled Republicans would want for their support. Unfortunately for the Democrats, I don't think there's much. Republicans are already, in the deal, getting their key policy goals fully met on taxes for upper-income constituents. Beyond that, I strongly suspect the major calculus for wavering Republicans in the House and Senate will be what Rush, Beck, Palin, and other Republican opinion leaders say about the deal -- and there's not much that the White House or Congressional Democrats can offer to get them to go along. Their logic is to oppose the White House no matter what (it's good for ratings!), rather than to bargain for specific policies, and in this post-(Bob) Bennett era, it's likely that Congressional Republicans are scared of crossing them.
On the other hand, specific Members of Congress, Republicans included, may have specific, district-related interests that Obama and the Democrats can, in fact, meet. So my guess is that the way to sweeten the deal all around is perhaps going to be wholesale, on the Democratic side, and retail on the GOP side.
Oh, and I'm not sure where to fit this in, but I do recommend an excellent post by Steve Benen today. My feeling is that it he's right that, at the end of the day, liberals should want this thing to pass (and, in fact, I think the same is true for conservatives -- I think this is as good a deal as they're likely to get on their priorities). What I'd add is that even if it makes sense at the end of the day to want a bill to pass, that doesn't mean (1) that you shouldn't bargain over the details until the last minute, and (2) that everyone who wants a bill to pass also wants to be the one voting for it. And what makes these things so interesting to watch is that while negotiations continue it's never certain who is just trying to cut the best deal possible, who really has serious demands that can't be met, who intends to vote against it unless his or her vote is needed, and who is simply opposed.
All in all, it's going to be a fascinating process, and a great test of Barack Obama's presidential skills.