He's too young (50 years old), and appointing someone who comes down hard on one side of the ideological divide in the judiciary for thirty or more years is bad for the country.
Being able to know in advance how a judge is going to rule on an unsettled question of law, because you know the judge's ideology, is a very bad thing for the law. Because that division in legal interpretation of the Constitution tracks some of the left-right political divisions, it has broad political implications that threaten the essence of an unbiased, non-political judiciary.
I don't know how to solve this problem, which comes down to ideological litmus tests for supporting or opposing judicial nominees. I do know how to reduce the problem though: reduce the stakes of each individual appointment by not having it last so long. What we should really have is term limits for Supreme Court Justices, and possibly other judges, which is something Roberts himself has supported. Absent a Constitutional amendment to put this in place, the alternative is to appoint older nominees. If the average stay on the Court is 20 years or less, there will be enough turnover so that the opposition political party can expect a good chance to nominate their own Justices when they retake the presidency, and the nomination battles will be less intense.
This is just a variation on Roberts' own idea. He should do something else for a while, and then maybe be an appropriate nominee by a conservative president when Roberts is over 60.
key: politics, law