The folks at Kaiser Health News pretty much nailed it.
In order to qualify for federal financing, the state exchanges must be able to ensure that premium-support recipients are living U.S. citizens -- a requirement to protect against fraud -- and are not felons. They must have household incomes between 133 percent and 400 percent of the federal poverty line (about $90,000 for a family of four). They also cannot be recipients of other health benefits from another source, such as an employer.
Logistically, these requirements present a massive challenge. For the first time, secure data feeds from the Departments of Homeland Security (establishing legal immigrant or US citizen status), Justice (for felon history), Treasury (for tax return information to impute income) and the Social Security Administration (establishing that the recipient is not deceased) would have to be combined. These data feeds would then have to be securely coordinated by the Department of Health and Human Services. There is no history of these agencies ever bringing their data together at this scale. It would qualify as the largest IT integration project in U.S. history.
Next, all 50 states would have to integrate this data into 50 different versions of a Travelocity.com for health insurance -- all while seamlessly shifting millions of recipients back and forth between private insurance and public programs like Medicaid and CHIP; allocating subsides; and collecting insurance premiums.
Only the folks from Washington could conceive something so complex as to be impossible to implement.