In March, Henry Chao, the chief digital architect for the Obama administration's new online insurance marketplace, told industry executives that he was deeply worried about the Web site's debut. "Let's just make sure it's not a third-world experience," he told them. Two weeks after the rollout, few would say his hopes were realized. For the past 12 days, a system costing more than $400 million and billed as a one-stop click-and-go hub for citizens seeking health insurance has thwarted the efforts of millions to simply log in. The growing national outcry has deeply embarrassed the White House, which has refused to say how many people have enrolled through the federal exchange.