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Washington • Seven million people signed up, so there is an appetite for President Barack Obama's health care law, but that doesn't guarantee success for the country's newest social program.

Big challenges are lurking for the next enrollment season, which starts Nov. 15. Chief among them are keeping premiums and other consumer costs in check, and overhauling an enrollment process that was advertised as customer-friendly but turned out to be an ordeal.

The source of the pent-up demand that propelled health care sign-ups beyond expectations could stem from the nation's new economic reality: a shrinking middle class and many working people treading water in low-paying jobs.

It could take the rest of the year to sort out how many uninsured people have actually gotten coverage, the ultimate test of Obama's law.

An ongoing Gallup survey has shown a steady drop in the share of Americans without insurance since Jan. 1, when the law's main coverage expansion took effect. Those numbers should improve because many people still can take advantage of extensions granted by the administration, and because those eligible for the law's Medicaid expansion can apply at any time.

Still, vindication for Obama's law isn't guaranteed. Among the top challenges:

Affordability • Health insurance premiums tend to go up every year, so the question now is how much higher in 2015.

An improving economy and the law's taxes on insurers will tend to push up premiums. Mechanisms in the law to assist insurers with a disproportionately large share of high-cost patients will push down premiums.

The big unknown is what economic bets insurers made when they jumped into the markets created by the law. If they were conservative and figured a big share of costly cases among the newly insured, that would take some pressure off premiums for next year.

Another important affordability issue has to with deductibles and copayments that consumers have to pay when they use their insurance benefits. Many of the new plans have high out-of-pocket costs, a trade-off for keeping premiums low.

Overhauling the sign-up process • One of the law's main goals was to take the mystery out of purchasing insurance.

But even when the websites are working, the insurance exchanges are anything but easy to navigate.

Finding out what hospitals and doctors are in particular plans requires additional work. Also, experts say it's really difficult to get to a true bottom-line estimate that includes premiums and expected cost-sharing.