The first reason for a positive vote for Sen. McCain for President (or if you prefer, a negative vote against Sen. Obama), is their vastly different outlook on the power and uses of the judiciary, particularly the Supreme Court. During the next Presidential term, there are likely to be two to three Supreme Court nominations. Sen. McCain, as he has notably said, is a Federalist. While not a strict Constructionist, he believes in a balance of power between the three federal branches, and that the federal power must be balanced against that of the states (and in many ways subjected to the power of the states). While he has said that he would not impose ideological litmus tests on judges, for which I applaud him while perhaps ruing the outcome for the pro-life movement, he would appoint judges that looked on their power as more restricted. That is, they are not there to make law but to adjudicate the merits of a case UNDER the law.
Sen. Obama tends to believe that the court's power is more fluid and encompassing. The recent release on the internet (see the download below) of a radio interview he gave while a State Senator in IL demonstrates his view that the Supreme Court is more the supreme decider of justice, rather than arbiter of it. Hence the Supreme Court could become the agent of redistribution of power and wealth to right historic wrongs and injustices, particularly if they have lingering effects.
Don't mistake me - justice is important. But the legislative branch is the means by which we change standards and law. And redistribution is the wrong way to right a wrong that stretches back in history. We can not afford to repay every debt, no matter how egregious. What we can do, is ensure economic growth and power are accessible to all who are willing to work for them and contribute to society. And that is not a job for the Supreme Court. Do you want to avoid and 'activist' court intent on imposing the will of the President for justice and redistribution? Vote for McCain.