Replacing Souter

The retirement of Justice Souter brings focus to the reason that we should be glad that Barack Obama defeated John McCain. Appointing Supreme Court Justices is probably the most significant longterm act of any presidency, and the current court is the legacy of the presidents in our recent past. If McCain had won the election his appointment choice could have changed the basic philosophy of the court. One more staunch conservative vote would have changed the balance for many years.
Obama's choice won't.
The over-simplified reason is the current court balance. The four staunch conservatives (Thomas, Scalia, Alito, and Roberts) will be (barring the unforeseen) on the court for many more years, whereas the "liberals" are getting toward the end of their careers. Souter is now considered a "liberal" but only in a very "moderate" way. The balance still swings on Kennedy, a mostly conservative (but sane) jurist, so there is a sort of "balance" that will continue for the foreseeable future.
No matter how "liberal" Obama's choice is (I was hoping for Angela Davis), it's not going to change the "balance". The right wing will yell and scream about whoever he chooses, but it's a whole lot of sound and fury signifying nothing. The court will still be conservative, with a hint of moderation.
I'm guessing that Obama will choose someone (a woman) who is moderately liberal, and that confirmation will be relatively smooth. A "safe" choice (unless he gets a 'reverse Souter').
We'll hear a lot about the conservative "culture" issues of abortion and gay marriage, but the status quo will remain the same regardless of who Obama nominates. This isn't the vote that can change the game, so the fight will be mostly for show.
The only real court change will come when Kennedy or one of the conservatives leaves. Or we have another wacko conservative president (McCain-ish) who can push the court farther right. But Obama can't push the court further left with this appointment. 5 to 4 is still 5 to 4.