Caroline F. Pearson, a vice president of Avalere Health, a consulting company that has analyzed hundreds of plans, said: “The premiums are lower than expected, but consumers on the exchange will often face high deductibles and high co-payments for medical services and prescription drugs before they reach the cap on out-of-pocket costs,” $6,350 for an individual and $12,700 for a family.

Those limits provide significant protection, even though those sums are substantial for most consumers. In addition, the federal website, HealthCare.gov, informs people that they may qualify for subsidies to reduce their out-of-pocket costs if their household income is below 250 percent of the federal poverty level, meaning that it is less than $28,725 for an individual or $48,825 for a family of three.

These “cost-sharing reductions” are available for a specific kind of midlevel plan known as a silver plan. People with lower incomes can get more help with out-of-pocket costs, but only if they choose silver plans.

Mr. Dorn said the government had not done much to inform people of these potential savings. “Consumers are giving up cost-sharing reductions of enormous value if they enroll in a bronze plan because it has the lowest premium,” he said.

Plans in the marketplace are separated into four categories — bronze, silver, gold and platinum — indicating the generosity of coverage, or the share of costs paid by insurance for an average enrollee.

Many people buying insurance on the federal and state exchanges are expected to qualify for subsidies. But in the first month, for reasons that are not clear, only 30 percent qualified. The others must pay the full premium and will be subject to the full deductible.

Most people shopping in the exchanges are expected to choose bronze or silver plans, which provide less generous coverage than most employer-sponsored plans.