Fatal flaw #1: The penalty imposed for not buying a policy is very likely to be less than the cost of insurance for a great many people. This, combined with the requirement that insurance companies may not deny coverage to anyone with a pre-existing condition, means that a large number of people will forgo signing up for a policy, knowing that they a) will save money and b) can always sign up for insurance if they turn out to develop a serious medical condition. Thus, the actual revenues will far way short of projections.
Fatal flaw #2: The government has no ability to enforce the penalty for noncompliance.
Fatal flaw #3: Mandating that people buy a health insurance policy simply because they are alive is arguably unconstitutional. It is also a way of hiding the fact that young people will effectively be paying a huge new tax in order to subsidize older people.
Fatal flaw #4: Regulating the price which insurance companies must charge for policies, coupled with a requirement that companies must rebate to their customers the amount by which their loss ratios fall below 90%, effectively turns these companies into government-run enterprises and would likely result in the effective nationalization of the healthcare industry. That is a violation of the Fifth Amendment, and of a Supreme Court requirement "that any firm in a regulated market be allowed to recover a risk-adjusted competitive rate of return on its accumulated capital investment."
Fatal flaw #5: A government-imposed restructuring of the healthcare industry can't possibly improve our healthcare system, and is extremely likely to make it worse. As Don Boudreaux has noted, "Trying to restructure an industry that constitutes one-sixth of the U.S. economy is ... so complicated that it's impossible to accomplish without risking catastrophic failure."
Fatal flaw #6: In cases wherein companies find that complying with the law would result in large increases in healthcare premiums that would threaten employees' access to a plan, the Dept. of Health and Human Services may grant a waiver to the company. As evidence of the first five fatal flaws accumulates, and as healthcare insurance companies continue to raise premiums to pay for the unintended consequences of government attempting to regulate an entire industry and hundreds of millions of people, more and more companies are likely to apply for waivers. At some point the whole edifice will come crashing down of its own weight.