The Obama administration's last-ditch efforts to get people to sign up for insurance through the insurance exchanges paid off as the total enrollment numbers exceeded 7 million by the March 31 open enrollment deadline.

The enrollment data surprised nearly everyone. Just weeks before the deadline, the Congressional Budget Office revised its estimate of new enrollees down to 6 million because of the numerous technical glitches that threatened to derail the program at the time of its launch last October.

But while proponents of the Affordable Care Act had reason to celebrate, there are still many unknowns that will determine the long-term success of the law.

"Part of the issue is we're talking about something with many moving pieces," said Bill Melville, market analyst for Decision Resources Group, a healthcare research and consulting firm based in Burlington, Mass. "To bend the medical cost curve is a long-term project ... and it could be three or four years before we really have a grasp on where things are going."

A concern from the administration has been whether enough young, healthy individuals will sign up and diversify the risk pool enough to keep rates low. The most recent enrollment numbers show about 27 percent of the plans sold on the exchanges were for individuals between ages 18 and 34.

Mike Stahl, senior vice president at HealthMarket Insurance Agency, said historically his company's data showed about 30 percent of plans covered people under age 35. The age mix hasn't changed much in the post-ACA world, he said. HealthMarket is an insurance company with more 3,000 independent agents nationwide who are certified to sell plans on and off the exchanges.

In the short-term, Melville said, insurers will be looking at how people who recently gained coverage end up using it. "Is it going to result in a rush on medical services and doctors being overwhelmed? That's one of the big unknowns here."

Insurers have until May to decide whether to participate in the exchanges for 2015, which won't leave them much time to analyze use patterns, Melville said.

There's also the potential for a backlash from those who bought plans but didn't quite understand how they worked.

Stahl said his company has already received numerous calls from clients who bought plans and were surprised to learn that they could not see the doctor down the street because he or she was out of network. Many are unfamiliar with insurance concepts such as networks, formularies, deductibles and co-pays. Agents have been key in educating them, he said.

Another part of the education process is teaching enrollees how and when they must pay for coverage, Stahl said. People new to insurance might not understand how it works.

The law allows a 90-day grace period for premiums to be paid, but insurers only cover services performed during the first 30 days. Policyholders who don't pay after 90 days will have their plans dropped, and physicians aren't paid for services from the last 60 days. Physicians have expressed concern they will be left holding the bag for services rendered during this grace period.

The Blue Cross Blue Shield Foundation reported that between 80 and 85 percent of exchange plan enrollees are paying their premiums. Stahl has seen similar numbers reported by the 140 carriers his company represents.

"That's not to say we're not concerned," he said. "We're working with customers to ensure they understand the process. We didn't want any confusion about how to pay for insurance."

There will continue to be speculation from both proponents and opponents on the long-term impacts of the ACA. But the lessons learned from the last week of open enrollment can all be boiled down to one: expect the unexpected.

To get an idea of the "enormous acceleration" of activity that caught everyone by surprise, Stahl said HealthMarket's call center gets about 1,000 calls a day on average. The frequency increased to 20,000 per day a week before the open enrollment deadline.

On March 31, the call center took 75,000 calls.