This op-ed by law professor Geoffrey Stone lays out the issues in picking a new Supreme Court justice pretty well:
Rulings by conservative justices in the past decade make it perfectly clear that they do not “apply the law” in a neutral and detached manner. Consider, for example, their decisions holding that corporations have the same right of free speech as individuals, that commercial advertising receives robust protection under the First Amendment, that the Second Amendment prohibits the regulation of guns, that affirmative action is unconstitutional, that the equal protection clause mandated the election of George W. Bush and that the Boy Scouts have a First Amendment right to exclude gay scoutmasters.
Whatever one thinks of these decisions, it should be apparent that conservative judges do not disinterestedly call balls and strikes. Rather, fueled by their own political and ideological convictions, they make value judgments, often in an aggressively activist manner that goes well beyond anything the framers themselves envisioned. There is nothing simple, neutral, objective or restrained about such decisions. For too long, conservatives have set the terms of the debate about judges, and they have done so in a highly misleading way. Americans should see conservative constitutional jurisprudence for what it really is. And liberals must stand up for their vision of the judiciary.
Of course, it doesn't really matter because Obama is going to pick a conservative anyway.