Some on the left are questioning whether Elena Kagan will be the type of progressive Justice they've been waiting for. The answer, I think, turns on what one means by progressive.
Justice William Brennan was one type of progressive justice. He broadly construed the Constitution to protect those rights he thought necessary for individual dignity, including rights that had little clear basis in either the Constitution's text or history. He read the Constitution, for instance, to outlaw the death penalty even though provisions of the Constitution refer approving to capital punishment. He also broadly read into the Constitution protections for new rights, like abortion. Many progressives wanted Obama to pick someone like Brennan, who would aggressively assert a liberal vision of American and do anything to make that vision into reality.
Kagan will almost certainly not be that kind of progressive. While I would expect she'd vote to uphold Roe v. Wade and maybe even vote for marriage equality, she is likely to be more moderate than a Brennan. She is almost certain to avoid creating a whole new realm of rights to achieve economic equality -- the latest bold vision of progressives, as described by Noah Feldman in the this past Sunday's New York Times Magazine.
But that doesn't mean she won't be a progressive. She may be what we might call a pragmatic progressive. Many progressives realize that the activism of the Warren Court in general, and Brennan in particular, had a few downsides. Their bold decisions fueled a backlash, one that helped pave the way for a resurgent Republican Party and the rise of a forceful conservative legal movement. It's hard to obtain progressive reform in that environment, and many such reforms by their nature have to come from the elected branches rather than the courts.
Pragmatic progressives still seek to fulfill the Constitution's promise of equal citizenship, but recognize that we need more than just bold decisions that are easily characterized as judicial activism to accomplish those goals. We need legislatures, interest groups, and the marketplace itself to help us. Plus, with a conservative Supreme Court, the judiciary can't be relied upon. Given the prevalence of pragmatism in today's left-of-center legal scholarship, I would expect that Kagan will lean towards this kind of progressivism.