Thanks to the Supreme Court and Barack Obama's re-election, the Affordable Care Act—"Obamacare" to foes and a few of its friends—isn't going away. The issue now is how it will work.

Even by Washington standards, implementing this law is extraordinarily complex. The federal government last year issued 70,000 pages of guidance, including 130 pages on the look of websites for new marketplaces where many will shop for insurance.

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Mr. Obama barely mentioned the law in his State of the Union address Tuesday. If it works as he hopes, he will have secured his legacy and solved the long-festering problem of the uninsured—though not the companion problem of rising costs. If the law flops or provokes a backlash, a future president will be forced into more radical reshaping of the health-care system.

Implementing the act turns on what Paul Keckley, head of the Deloitte Center for Health Solutions, calls "the four major hanging chads."

A quick look at each:

What will consumers do?

Most will do pretty much what they do now. About 55% of Americans of all ages get health insurance through an employer; another 32% through a government program. For most, not much will change, though workers are likely to pay more for health care as employers pass along costs. Also, the law will require employers who offer skimpy benefits to provide more robust ones.

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The challenge is to prompt one group of consumers to change: the 18 million 20- and 30-somethings who don't have health insurance. The arithmetic of Obamacare depends on getting more Americans to buy health insurance. If the young and healthy don't show up, the math doesn't work—and the cost of insurance for those who do shop in the new exchanges will be higher. That's why there is a high-profile campaign in the works to recruit young people.

What will employers do?

Mostly wait and see. Even employers flirting with getting out of the benefits business or giving workers a fixed sum and letting them shop for insurance won't move quickly.

Implementing the Affordable Care Act is extraordinarily complex. Above, customers get information at UnitedHealthcare store in New York City. Bloomberg News

One provision already appears to be having unwelcome, unintended consequences. It requires employers with more than 50 workers to offer insurance to anyone who works 30 hours a week or more. That gives fast-food, retail and other employers who rely heavily on low-wage, part-timers an incentive to keep workers to 29 hours or less; word is that many already are doing so. (It won't be long before some Democrat in Congress proposes lowering the threshold to 20 hours.)

Smaller firms have reason not to expand their workforces above 50, or to game the system by subdividing themselves.

The Congressional Budget Office estimates that about 8 million fewer workers, about 5% of the total, will get insurance through employers five years from now than would have been the case without the Affordable Care Act.

But no one really knows how many employers will find ways to drop coverage or to structure benefits so sicker, costlier workers get insurance at the new exchanges.

What will states do?

They have until Friday to decide whether to create an exchange or let the federal government set one up. New York and California already are committed to fully running their own exchanges; Texas and Georgia have signaled they won't take on any of the tasks. It isn't clear how much help the abstainers will offer Washington in crafting the marketplaces and recruiting customers. State resistance would be an obstacle to an already tough task.

And then there is Medicaid, the state-federal program for the poor. The law substantially expands eligibility, at Washington's expense for the first three years, but the Supreme Court ruled that states don't have to go along. CBO projects that in the next five years, Medicaid rolls will grow to 45 million from 36 million.

About half of the governors (including six of the 30 who are Republican) have decided to expand their Medicaid programs; turning down federal money is hard.

But about half say they won't (including 13 Republicans) or are still on the fence (including 11 Republicans). States that don't expand Medicaid likely will have more uninsured, which means, among other things, that health-care providers and employers who do offer insurance will, effectively, pick up the cost of caring for the uninsured when they do get care.

What will health-care providers do?

Merge and grow bigger. The law encourages the integration of hospitals, doctors and nursing homes. It takes aim at the underlying problem: The U.S. has evolved the developed world's most inefficient health-care delivery system, one that too often rewards volume of care and not quality. Integrated health providers such as Kaiser Permanente and Geisinger Health System are seen as models of low costs and high quality.

But there is a risk or—if you talk to insurers—a nightmare. The proliferation of hospital mergers and hospitals' appetite for buying doctors' practices—in part to assure a steady stream of patients to fill hospital beds—could create local monopolies that raise prices without increasing efficiency. "Historically," says Deloitte's Mr. Keckley, "hospital consolidation hasn't reduced costs."

The next couple of years will bring the biggest changes to the American health-care system since the advent of Medicare and Medicaid in 1965. We're about to find out if they're for the better.

Corrections & Amplifications
There are 30 Republican governors. An earlier version of the column incorrectly said there were 29.