Unanimity and the Roberts Court:
Here's an interesting discussion on that topic with Jeff Rosen, Kathleen Sullivan, Doug Kmiec, Ken Starr, and Vikram Amar. (via Howard) A few thoughts in response:

  First, I don't agree that we have an unusually divided court. I think the current Supreme Court is less divided than the Court has been at most times in its history. What's unusual about the current Court, from a historical perspective, is that it pretty much only takes cases when there is a clear circuit split. That means the Justices tend to take technical cases that are very hard, rather than grand questions of the direction of constitutional law. It's a pretty different docket today than you had 30 or 50 years ago. This magnifies divisions, as two people who basically agree on a topic can always disagree on particulars.

  Second, I think Roberts' interest in unanimity mostly reflected his interest in keeping the role of the Supreme Court fairly narrow. Roberts doesn't want the Supreme Court to make enormous bold leaps, and unanimous decisions are usually decisions that don't rock the boat. As you get to a more divided Court, the chances of bold leaps tends to increase. So Roberts may have been thinking that he could limit the power of the Court by pushing the Justices to adopt narrower, unanimous ways of deciding cases.

  Finally, I think the recent frustration apparent among the four liberal Justices is partly the result of Roberts being much more party-line than many expected. Roberts cultivated an image as the principled minimalist, but last Term he consistently voted in a conservative direction. If a coin flip always comes up heads, after a while you're gonna wonder about the coin. I would guess the liberal Justices began to think they'd been "had," and their frustration at least partially reflects that.