In this post, Menachem Mendel compares the declared values by which Obama would nominate Supreme Court justices with the role of the poseik as described by Prof. Sperber. I think it's right on the money (and didn't even need Prof. Sperber to tell me so). The greatest poseik is the one who combines knowledge of the law with sensitivity to the plight of the petitioner.
In many recent halakhic issues that have been heavily debated (Shira Hadasha-style minyanim, conversion, etc.) the two sides tend to pit these as the issues - the liberal side contending that the conservative side lacks compassion, and the conservative side contending that the liberal side lacks fealty to halakha.
Ultimately, though, both sides are vital. The greatest poskim understood that, implicitly if not explicitly. There are many of them out there today as well, but they are unfortunately not as visible as the ones on one side of the debate or the other.