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.

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.