“It is three years before most of the new health-care law kicks in, but already some of America’s largest employers are peppering the Internal Revenue Service with concerns that making the changes will be far more complex than they anticipated. At issue is one of the law’s central requirements: employers with 50 or more full-time workers must offer affordable insurance or pay a penalty. It sounds simple enough. But in crafting the rules, the IRS and two other federal agencies are now tackling basic yet messy questions, such as who counts as a full-time worker and how do companies measure whether insurance is ‘affordable.'”