I like John Roberts. I think he'll make a fine justice. But I oppose his nomination for chief justice for two reasons:
1) President Bush skipped over some tenured justices on the Supreme Court itself. I realize that the Chief's job is mainly administrative and Roberts should be able to fill that role adequately. But he has little experience in the judicial role. There are many more justices on the Court (Scalia, Thomas, etc.) who were passed over by the President when he decided to appoint a novice. It looks pretty bad when we so completely lack confidence in the sitting members of the court that we want to put a 50 year old with limited experience on top of them.
2) Roberts will now be perceived as filling the Rehnquist seat. That should make his confirmation a no-brainer, even among the far-left. It's hard to criticize Bush for appointing a Rehnquist clerk to fill his seat. No unbalancing of the Court going on here.
But that's precisely the problem. Roberts is a fine conservative, but he's not an originalist, and therefore not in the mold of Scalia and Thomas, Bush's paradigm justices. He would have been a good choice to replace O'Connor because he's not that far to the right, has no extreme positions, is eminently qualified, and is well liked. If he replaced O'Connor, the Court would shift more to right, which makes sense given the conservative control of the other two branches of the government.
But if he replaces Rehnquist, Bush now has to find someone with similar characteristics to fill the O'Connor seat. Bush cannot nominate an originalist, because that will be a far greater shift. So most likely he'll try to find someone like Roberts, and if he succeeds the Court's shift will be minimal. But it's even more likely Bush will appoint someone closer to O'Connor because not everyone is Roberts. So there will no shift at all. Two vacancies and no ideological movement is bad for a Republican president who opposes the "living Constitution" theory.