As people become aware of the differences among the exchanges, “some of the laggard states are going to end up changing,” said Ron Pollack, the executive director for Families USA, a consumer advocacy group that supports the law.

Whether the law ultimately accomplishes its aim of making the insurance markets nationwide more competitive — and plans more affordable — will only become clear over time. Experts expect some insurers to drop out after a year or so, while some other companies may decide to enter, depending on how the markets evolve. Insurers will have to figure out how to offer plans that most people can afford but still provide coverage to those with expensive medical conditions — and, for investor-owned plans, how to make a profit in the meantime.

“A rush to judgment will be just that,” said Dan Mendelson, the chief executive of Avalere Health, a consulting group. “It’s not going to be possible in 2014 to make a strong valid judgment of whether the exchanges are working or not.”

Insurers already active in the market are the most likely to show up on the exchanges. Blue Cross plans, for example, have already established relationships with local hospitals and physician groups, as well as state regulators. “We don’t have to recreate the wheel because the Blue plans are already there,” said Daniel J. Hilferty, the chief executive of Independence Blue Cross, a nonprofit headquartered in Philadelphia.

In California, Anthem Blue Cross, Health Net, Kaiser Permanente and Blue Shield of California will remain big players. Most likely to be missing from any given exchange are many of the national insurers, whose business is focused mainly on providing coverage to workers through their employers — companies liked UnitedHealth Group, Aetna and Cigna.

WellPoint, which operates Blue Cross plans in 14 states and is the nation’s largest provider of individual and small business policies, has little choice but to compete because many of its customers will be buying insurance on the exchanges.

But the other companies may delay entering any given exchange until they see a real chance to gain customers. Given the uncertainty over how well the exchanges will function, and whether enough healthy people will enroll, insurers are likely to enter only those markets where they already have a sizable number of existing customers.