I think the thing that's very worrisome is that she has a very expansive view of the Commerce Clause, and I find that she's ignorant of the Constitution's limitation of that, especially what our Founders wrote.
Coburn is probably right to surmise that Kagan "has a very expansive view of the Commerce Clause" (as does the justice she is replacing and as any nominee chosen by Barack Obama was apt to), and I agree that it's worrisome. But it's unfair to describe her as "ignorant" of the Framers' intent or the original understanding of the clause. More likely, she does not think those considerations should be decisive in determining the federal government's powers.
In any case, Coburn himself "has a very expansive view of the Commerce Clause" when it serves his purposes. Another subject on which he grilled Kagan was "partial birth abortion," which he successfully sought to prohibit throughout the country. In 2007, after the Supreme Court upheld the federal Partial-Birth Abortion Ban Act, Coburn welcomed the decision and bragged that he was "one of the original authors" of the bill. Guess which enumerated power Congress supposedly was exercising when it passed Coburn's law.
The ban applies to "any physician who, in or affecting interstate or foreign commerce, knowingly performs a partial-birth abortion." As Independence Institute scholar David Kopel and University of Tennessee law professor Glenn Reynolds noted in a 1997 Connecticut Law Review article, this language is baffling "to any person not familiar with the Commerce Clause sophistries of twentieth century jurisprudence....Unless a physician is operating a mobile abortion clinic on the Metroliner, it is not really possible to perform an abortion 'in or affecting interstate or foreign commerce.'" These are precisely the sophistries that Coburn, one of the few members of Congress who even pays lip service to the doctrine of enumerated powers, claims to oppose. Just as the Supreme Court should not have abused the Constitution to override state abortion restrictions in Roe v. Wade, avowed constitutionalists like Coburn should not abuse the Constitution to impose federal abortion restrictions.