Some people have been on the SCOTUS forever.
The year was 1895. Supreme Court Justice and Lincoln appointee Steven Field was asked by a colleague to retire. He was very old by this time and was plagued with what would be called Alzheimer's Syndrome. He refused, saying that he wanted to set the record held by Chief Justice John Marshall and told his colleague that to even ask that question was rude. The Justice mentioned that back in 1877, he himself had led an effort to get Associate Justice Robert C. Grier. Field snarled back ... "A dirtier deed I've never done."
When a person is appointed to the Supreme Court it is for life. Consider: John Paul Stevens, the senior member of the current court, is 88 years old. He was appointed by President Ford back in 1975 and has served almost as long as Field, Marshall, and his own immediate predecessor, William O. Douglas, who was appointed by Franklin Roosevelt in 1939. Douglas, in turn, was appointed to replace Louis Brandeis, who had served over a quarter of a century himself.
In other words, in a period of 92 years, only three people have sat in that particular seat. With modern medicine getting better all the time, Stevens' replacement could still be on the bench at the start of the World War II CENTENNIAL.
SCOTUS is, from time to time, a geriatric organization. Lifetime appointments and the power of the job, promote tenures of decades and decades. Before the retirement of Sandra O'Conner and the death of William Rehnquist a few years back, the court's makeup was static for twelve years. The court is divided between generations, and the older one may retire very soon, giving the next president a chance to make several important appointments that will affect the country forever.
As Chief Justice Charles Evens Hughes once said: "The Constitution is whatever the Supreme Court says it is." This is almost true. From the 1870s, when Field and Greer were in their prime, the Supreme Court actually declared the 14th and 15th amendments null and void (US vs. Reece, 1875) and the court didn't really start cleaning up the mess for another 70 years.
What kind of Supreme Court Justices and Appellate court judges would McCain or Obama appoint? Would the 14th Amendment, which makes the bill of rights binding on State governments, be respected? Several justices, such as Scalia and Alito, don't think so. Then there's the question of Federalism. How independent are the fifty states, five territories and DC vis-à-vis the federal government? Can Delaware go it's own way in regards to human rights? If gay marriage is okay, then how about bigamy? It's a genuine question that is going to be brought up, whatever you think about Gay rights. Then, there's the perennial issues of wiretapping and break-ins. How much power does the President really have?
That may sound a bit arcane, but short of starting a war, nominating a Supreme Court Justice is the most important thing a president can do. Roe vs. Wade is jus the least of it.