Update: Philip Klein posts the "blistering" conclusion to the dissent here:
The Court today decides to save a statute Congress did not write. It rules that what the statute declares to be a requirement with a penalty is instead an option subject to a tax. And it changes the intentionally coercive sanction of a total cut-off of Medicaid funds to a supposedly noncoercive cut-off of only the incremental funds that the Act makes available.
The Court regards its strained statutory interpretation as judicial modesty. It is not. It amounts instead to a vast judicial overreaching. It creates a debilitated, inoperable version of health-care regulation that Congress did not enact and the public does not expect. It makes enactment of sensible health-care regulation more difficult, since Congress cannot start afresh but must take as its point of departure a jumble of now senseless provisions, provisions that certain interests favored under the Court’s new design will struggle to retain. And it leaves the public and the States to expend vast sums of money on requirements that may or may not survive the necessary congressional revision.
The Court’s disposition, invented and atextual as it is, does not even have the merit of avoiding constitutional difficulties. It creates them. The holding that the Individual Mandate is a tax raises a difficult constitutional question (what is a direct tax?) that the Court resolves with inadequate deliberation. And the judgment on the Medicaid Expansion issue ushers in new federalism concerns and places an unaccustomed strain upon the Union. Those States that decline the Medicaid Expansion must subsidize, by the federal tax dollars taken from their citizens, vast grants to the States that accept the Medicaid Expansion. If that destabilizing political dynamic, so antagonistic to a harmonious Union, is to be introduced at all, it should be by Congress, not by the Judiciary.
The values that should have determined our course to- day are caution, minimalism, and the understanding that the Federal Government is one of limited powers. But the Court’s ruling undermines those values at every turn. In the name of restraint, it overreaches. In the name of constitutional avoidance, it creates new constitutional questions. In the name of cooperative federalism, it undermines state sovereignty.
The Constitution, though it dates from the founding of the Republic, has powerful meaning and vital relevance to our own times. The constitutional protections that this case involves are protections of structure. Structural protections—notably, the restraints imposed by federalism and separation of powers—are less romantic and have less obvious a connection to personal freedom than the provisions of the Bill of Rights or the Civil War Amendments. Hence they tend to be undervalued or even forgotten by our citizens. It should be the responsibility of the Court to teach otherwise, to remind our people that the Framers considered structural protections of freedom the most important ones, for which reason they alone were embodied in the original Constitution and not left to later amendment. The fragmentation of power produced by the structure of our Government is central to liberty, and when we destroy it, we place liberty at peril. Today’s decision should have vindicated, should have taught, this truth; instead, our judgment today has disregarded it.
For the reasons here stated, we would find the Act in- valid in its entirety. We respectfully dissent.
Here's the entire section of the Scalia, Kennedy, Alito, and Thomas dissent on why the mandate is not a tax:
The Government contends, however, as expressed in the caption to Part II of its brief, that “THE MINIMUM COVERAGE PROVISION IS INDEPENDENTLY AUTHORIZED BY CONGRESS’S TAXING POWER.” Petitioners’ Minimum Coverage Brief 52. The phrase “independently authorized” suggests the existence of a creature never hitherto seen in the United States Reports: A penalty for constitutional purposes that is also a tax for constitutional purposes. In all our cases the two are mutually exclusive. The provision challenged under the Constitution is either a penalty or else a tax. Of course in many cases what was a regulatory mandate enforced by a penalty could have been imposed as a tax upon permissible action; or what was imposed as a tax upon permissible action could have been a regulatory mandate enforced by a penalty. But we know of no case, and the Government cites none, in which the imposition was, for constitutional purposes, both.5