Many Americans will receive subsidies for insurance, from what I understand roughly in the range of 6k to 12k. Many other Americans — namely those who already have health insurance — will not receive direct subsidies of this nature. Yet the subsidy-receiving and non-subsidy-receiving Americans will very often belong to the same income classes.
This disparity does not bother me personally (I have other worries about the subsidies), but I believe it will be very unpopular once it is publicly understood. One way or another, the "firewall" between the exchanges and the employer-supplied system will break down. Some people will want to spread the subsidies, others will want to limit them. Yet the former is budgetarily problematic and the latter will be politically difficult.
A second and related issue is that the differences in reimbursement rates — across private insurance, Medicare and Medicaid (highest to lowest) — will become a more pressing issue. For one thing, Medicaid patients will be crowded out by those buying private insurance on the exchanges, plus they will be crowded out by the growing number of Medicaid (and Medicare) patients. There will be pressure to fix this problem and the difference in rates will lead to growing supplier gaming, queues, quality differentials, and so on.
Over time, reimbursement rates across programs (insurance subsidies, Medicare, Medicaid) will converge to an increasing degree. Subsidies will be increasingly determined by income class rather than previous insurance history.
In the limiting case (I'm not suggesting we will get there), everyone will receive means-tested subsidized vouchers for regulated private insurance. In this strange way, Medicare and Medicaid could end up partially privatized and Ezekiel Emanuel — a voucher advocate — will end up being more influential than his brother Rahm. We will have to live with the problems of means-testing to a higher degree than today, but we will have something closer to a unified system, as do most other countries with universal coverage. There will be political pressure for compulsory health care savings, as they have in Singapore, to lower costs of finance.
It would be good if such vouchers could evolve in the direction of emphasizing catastrophic care and eventually they will have to.
Massive pressure will be put on such vouchers if either health care consumes 30-40 percent of gdp or income inequality continues to rise. In the former case, subsidies become increasingly expensive and involve extraordinarily high implicit marginal tax rates (earn more, your subsidy declines in value). In the latter case, it becomes increasingly difficult to ensure "near-equal" levels of health care access at feasible subsidy levels. Those pressure points are not unique to the Obama bill, but they become especially critical under the evolutionary scenario I am outlining. Perhaps we would give up the ideal of near-equal access, but that day is a few decades away.
Addendum: Here is Bryan Caplan's scenario, which means the bill will not work; my above post is assuming that problem is solved by raising the penalty. I am reading long lists of why the bill is so good but few proponents are analyzing that problem.