The leftosphere seems evenly split between those who oppose the individual mandate because it's not single-payer, and those who favor it as another brick on the road to single-payer.
Given the special-interest pressures that are the Achilles' heel of the individual mandate (and for that matter, the status quo), I do wonder what a single-payer system would look like in the U.S. I suspect it would have a rather different set of problems here than in countries that already have it. Special interests are a problem in any representative democracy, but the American system seems especially prone to cultivating them (though I don't say this with great confidence; unions seem especially powerful in other industrialized nations). In any case, in the inevitable trade-off between bloated expenditures and rationing-by-waiting, existing single-payer systems have generally chosen the latter. But I wouldn't be surprised if the U.S. went the other way, making even more healthcare available to everyone, paying munificent salaries to politically influential special interest groups in the medical field, and spending an even more outrageous amount of money on healthcare than we do now.