Justice Souter's reported decision to retire from the Supreme Court is a testament to his admirable lack of interest in the D.C. power scene. My guess (and I don't pretend that it's more than that) is that he's eager to return to his beloved New Hampshire to hike its mountains, read history and enjoy the quiet life.
What will Justice Souter be remembered for? No opinion of his comes to my mind except the joint opinion that he, Justice O'Connor and Justice Kennedy co-authored in 1992 in Planned Parenthood v. Casey. That joint opinion is significant not for its coherence or elegance (it has neither quality) but because it perpetuated Roe v. Wade's removal of the issue of abortion policy from the ordinary democratic processes — and it resorted to what Justice Scalia aptly called a “Nietzschean vision” of the judicial role in order to do so.
Justice Souter has been significant primarily for providing one of the five votes in favor of the “liberal” position on important cases.
The end result was not, as Souter and company contended, a resolution of the bitter national controversy over abortion, but the continued poisoning of American politics by the Court's power grab on that issue.
Beyond that opinion, Justice Souter has been significant primarily for providing, depending on where Justice Kennedy's vote has been, one of the five votes in favor of the “liberal” position on important cases (often inventing new rights, preserving activist precedents, or otherwise expanding judicial power) or, less often, one of the four votes dissenting from majority decisions that adopt the “conservative” position (which position is often simply to defer to democratic enactments). And, of course, his byzantine, often impenetrable, writing style is distinctive.
My guess is that Justice Souter will, in the end, be remembered most as President George H.W. Bush's worst mistake — a mistake, of course, that some celebrate and others (myself included) lament. Legal historians of the future may puzzle over how the same president could appoint both Justice Souter and Justice Thomas, and they will ponder how different — and, in my judgment, better — the state of American constitutional law and the quality of American politics would be if President Bush had not made the Souter mistake.