Obama's plan has three elements that are mutually incompatible:

1) Health insurance will be affordable.
2) Health insurance will be voluntary.
3) Insurers will not be able to deny coverage for pre-existing conditions.

You can't have all three at the same time. If people have the right to opt out of health insurance until they need it, while still being able to compel insurers to cover their pre-existing conditions at will, that's exactly what many will do.

I could, for example, do without health insurance (pocketing what I would have spent on premiums), "self-insuring" against small health care expenses (earaches, colds, rashes, etc.). I would know that if I ever get a severe problem or a chronic condition where opting out of health insurance becomes a losing proposition, I can opt in, secure in the knoweldge I'll take more out of the system than I have to put in. So, for example, if I break my hip and need an expensive surgery, or I'm diagnosed with AIDS or Diabetes, I can sign up and start taking out hundreds of thousands of dollars of value from the system, in exchange for my affordable healthcare premiums.

The problem there is glaringly obvious. If those who have health insurance are disproportionately those with extremely expensive ailments, premiums will need to be very high. You can't, as a society, get more money out of the health insurance program than you're putting in, so if the system is being less and less subsidized by healthy folks paying in more than they take out, then premiums need to rise higher and higher.

Obama's plan relies on a fundamental inability to grasp how insurance works: you pay in more than you take out, to insure against the risk that you may one day need to take out a whole lot more than you paid in. Give people the choice to pop in and out of the system based on their needs of the moment, without the peril of having pre-existing conditions denied, and you short-circuit the whole concept of insurance.

It's easy enough to spot the fatal flaw if you look at other mandatory insurance programs, like Medicare and Social Security. What if you could opt out of the Social Security program at will, but then opt in whenever you wanted, without having the Social Security system deny you benefits? The obvious play would be not to pay in at all until you were old enough to get a payout. It just wouldn't work. Same with Medicare. Or how about if you could do without auto insurance but then opt in at will and require your insurer to pay for any damage to your car, even if it predated your opt-in?

Clinton's system and McCain's system are not unworkable in that way. Each solves the problem realistically. McCain lets you opt out of insurance, but then forces you to suffer the consequences if you gamble and guess wrong, so that the more responsible people don't end up footing the bill for the health gamblers. You can't opt in and then expect the insurer to cover your pre-existing conditions. Clinton solves the problem by making coverage compulsory (as it is in Massachusetts' current system), so that nobody can pocket his premiums until he runs into trouble. Obama's system tries to ignore the problem, so that he can take cheap shots at both his opponents, while selling a fairy tale.