Judge. Thomas is presently a member of the U.S. Circuit Court of Appeals for the District of Columbia Circuit. The circuit courts are the second highest in the federal system. Only the Supreme Court is higher.
This means that if Thomas is confirmed, six of nine justices will be former federal judges who have been promoted. The federal judiciary is becoming just another bureaucracy. So the important "cases and controversies" that the Supreme Court considers are all looked at from the same professional perspective. (Of the 40 20th century justices prior to the present membership, only 13 were former federal judges.)
Yale. There have always been too many Ivy Leaguers on the Supreme Court (53 of 95 not counting current members), but this time it's ridiculous. If Yalie Thomas becomes a justice, then there will be five Harvard men (Souter, Kennedy, Scalia, Blackmun and William Rehnquist) and two Yale men (Thomas and Byron White) the court.
We are getting an English style "establishment" here, with old boys at the top whose societal and philosophical perspectives have been shaped by the same interlocking of locale, professors, lectures and texts.
Lawyer. Every justice has been a lawyer. Yet the Supreme Court doesn't really just deal in legalities or just settle cases. It makes sweeping grand policy. Five justices can approve or over-rule all other public officials' acts on the grounds that what has been done does or doesn't meet their understanding of what is fitting in light of our Constitution and history. Judges of lower federal courts, who do just settle cases, are required by law to be lawyers. But not Supreme Court justices.
Citizens who have been trained in other but appropriate disciplines should sit on this court. Former presidents, governors, members of Congress, the military, professors of history, politics and constitutional development, journalists (what the hell?). There are many non-legally educated Americans who have as great an understanding of the nation's constitutional history and workings as lawyers do. They would broaden the court's perspective. This is as important an element of diversity as geography, race and sex.
Trivia lagniappe: The Liberty Bell cracked 156 years ago today. It cracked tolling in mourning the death of Justice John Marshall. Marshall is almost universally regarded as the greatest justice. He created the American system of constitutional law, including judicial review. He did not go to Harvard or Yale (or even law school), and he was not a judge before he went on the Supreme Court. He was a lawyer. Well, two out of three ain't bad.