As lawmakers debate legislation aiming to extend coverage to the country's 46 million uninsured, one of the most sweeping proposals has so far stoked relatively little debate: a requirement that nearly all Americans carry health insurance. Subsidies for premiums would help low-income families gain coverage, while the prospect of fines would prod others to buy insurance. But some people show how difficult it could be to bring into the insurance pool the millions of consumers who make too much money to qualify for assistance, yet not enough to bear the full cost of new policies on their own.