In a post designed to calm progressives down, Ezra Klein succinctly explains the primary reason (in my opinion) to fear health care reform; political reality:
Democrats know full well that there are two plausible outcomes to the health-care reform process. Health-care reform will fail, dealing a huge blow to the Democratic Party and giving Republicans tremendous momentum as we enter the 2010 campaign season. Or health-care reform will pass, and Democrats will criss-cross the country touting the largest legislative accomplishment in decades. Republicans may still attack them on the plan. But attacking a historic legislative success is a whole lot harder than attacking a historic legislative failure. Republicans know that, which is why they want to kill the bill. Democrats know it too, which is why they won't let them.
If I'm reading this correctly then what we're likely to see play out is a Democrat party determined to push through a bill that will simply attract the most votes, and a Republican party hell-bent on killing it.
My hunch is that the former will most likely lead to an expansion of coverage, while the latter will distract conservatives enough to ensure that the bill fails to reign in costs.
Which suffice to say isn't a path to single-payer as much as it is a path to financial calamity.