According to First Among Equals, Ken Starr's history of the Supreme Court, one of the key features of the Rehnquist Court (which he claims could as easily be called the O'Connor Court) has been the reassertion of federalism. Certainly, Rehnquist and O'Connor were among the dissenters in the disgraceful Gonzales vs Raich, which eliminated the ability of the states to regulate intra-state commerce in marijuana.
This means two of the three dissenters to that appalling ruling will soon be off the Court. Their two replacements both have long experience of serving in the executive branch. We might reasonably expect, therefore, that the Roberts Court will show excessive deference to the federal executive, with Clarence Thomas relegated to the role Rehnquist held during the 1970s, of being lone dissenter in support of the Tenth Amendment.
I have argued before here and here that conservatives may come to regret O'Connor's retirement. I fear I will be proved right earlier than I imagined.