Here's the meat of the majority opinion:
We first conclude that the Act’s Medicaid expansion is constitutional. Existing Supreme Court precedent does not establish that Congress’s inducements are unconstitutionally coercive, especially when the federal government will bear nearly all the costs of the program’s amplified enrollments.
Next, the individual mandate was enacted as a regulatory penalty, not a revenue-raising tax, and cannot be sustained as an exercise of Congress’s power under the Taxing and Spending Clause. The mandate is denominated as a penalty in the Act itself, and the legislative history and relevant case law confirm this reading of its function.
Further, the individual mandate exceeds Congress’s enumerated commerce power and is unconstitutional. This economic mandate represents a wholly novel and potentially unbounded assertion of congressional authority: the ability to compel Americans to purchase an expensive health insurance product they have elected not to buy, and to make them re-purchase that insurance product every month for their entire lives. We have not found any generally applicable, judicially enforceable limiting principle that would permit us to uphold the mandate without obliterating the boundaries inherent in the system of enumerated congressional powers. “Uniqueness” is not a constitutional principle in any antecedent Supreme Court decision. The individual mandate also finds no refuge in the aggregation doctrine, for decisions to abstain from the purchase of a product or service, whatever their cumulative effect, lack a sufficient nexus to commerce.
The individual mandate, however, can be severed from the remainder of the Act’s myriad reforms. The presumption of severability is rooted in notions of judicial restraint and respect for the separation of powers in our constitutional system. The Act’s other provisions remain legally operative after the mandate’s excision, and the high burden needed under Supreme Court precedent to rebut the presumption of severability has not been met.
Here is my colleague Ilya Shapiro's statement on the ruling.