A number of names are being tossed about in the sweepstakes to succeed Justice John Paul Stevens, and the three front-runners at the moment seem to be Merrick Garland and Diane Wood, both federal appeals court judges, and Solicitor General Elena Kagan, former dean of the Harvard Law School. My own preference would be for Elena Kagan. This is not because of any particular virtue in Kagan, or defect in Wood and Garland--all three of whom, I'm sure, are eminently qualified to serve--but because Kagan, if appointed and confirmed, would be the first Supreme Court justice since Sandra Day O'Connor (1981) who had never been a federal appellate judge.
Beginning in the late 1960s it was President Richard Nixon's preference to appoint federal judges to the Court because he wanted some indication of a judicial 'track record' to guide his selections. He did not always abide by this rule--William Rehnquist (1971) was a senior Justice Department official--but since the Nixon years, tenure on the federal bench has become a bipartisan requirement for promotion to the Supreme Court. Lawyers are now routinely appointed to the federal appellate bench as a kind of minor league for the Court, and Court nominees are expected to have federal judicial experience. I have a happy memory of listening to Al Franken, in his Radio America days, angrily denounce President George W. Bush's ill-fated nominee, Harriet Miers, because she had never served as a federal judge!
As it happens, every current member of the Supreme Court (including Stevens) is a former federal appeals court judge. The fact is, however, that there is limited correlation between the quality (or ideological predictability) of justices of the Supreme Court and their experience as federal judges. Justice David Souter (1990), a reliable liberal vote on the court elevated by President George H. W. Bush, is the most dramatic recent example of this axiom. And there is considerable evidence that some of the most distinguished justices in our recent history were not only devoid of federal appellate experience, but had not even been judges of any kind before their appointment to the Court: Charles Evans Hughes (governor of New York), Louis Brandeis (lawyer in private practice), Felix Frankfurter (Harvard Law professor), and Hugo Black (senator from Alabama), to name a few.
No doubt, the ranks of the federal bench contain many worthy people, some of whom might make outstanding justices. But limiting any president's search to federal judges dramatically narrows the pool of available candidates, and often excludes people--senators, practicing attorneys, members of the Cabinet, constitutional scholars, state judges, civil administrators--whose distinction and experience are lost to the modern Supreme Court.