These are tough times for Wal-Mart, America's biggest retailer. Long accused of wrecking small-town America and condemned for the stinginess of its pay, the company has lately come under fire for its meanness over employees' health-care benefits. The charge is arguably unfair: the firm's health coverage , while admittedly less extensive than the average for big companies, is on a par with other retailers'. But bad publicity, coupled with rising costs, has stirred the Bentonville giant to action. Wal-Mart is making changes that should shift the ground in America's health- care debate.