With millions of people signing up, the law has cleared one long, difficult set of hurdles and has defied the darkest predictions of its critics. But that doesn't mean it's out of the woods entirely—it's just on to the next long, difficult set of hurdles.

What will happen to premiums?

It's too flip to say the national enrollment data "doesn't matter"—the whole point of the law was to cover millions of uninsured Americans, and whether that's happening matters very much. But the national total doesn't say much about of the law's sustainability or what happens to premiums next year.

Premiums will go up, because premiums go up every year. The size of next year's premium increases depends on enrollment and demographics in each state, and even within specific regions of each state.

Some states are faring better than others: California is well-positioned for 2015, with enrollment above expectations and a competitive market. On the other end of the spectrum are states like Mississippi, where there's only one insurance company in the state's exchange and the population is relatively unhealthy to begin with.

The demographic mix is pretty steady across all 50 states. Nationally, young adults—the most coveted enrollees—made up 25 percent of all sign-ups through the end of February. The percentage will probably improve in the final analysis; young people were always expected to sign up at the last minute.

Generally, if the risk pool in a particular market turns out older and sicker than expected, insurers are more likely to raise their premiums. We don't know precisely what insurers expected—they all priced their plans independently, and the law includes several programs designed to absorb any surprises and keep premiums as steady as possible. In competitive markets, insurers likely will try to keep increases to a minimum, but industry insiders caution that hikes are looking likely, at least in some parts of the country.

Will people like their plans?

So, enrollment in the first is decent, and there are tools to try to keep premiums in check, which will help enrollment continue to grow. The administration will probably do pretty well at getting people into the system. The biggest challenge for the Affordable Care Act, healthcare experts say, is whether people like what they find.

"The biggest question by far is how people feel about that coverage, and whether they think it's a good deal … and what is the proportion of winners to losers," Altman said.

The "losers"—people who had their plans canceled and must pay more for an equivalent policy that might not include their doctor—have gotten more attention so far. The "winners"—people with preexisting conditions who were denied or priced out of health insurance until now—will likely get more attention as more of them actually gain coverage.