Chief executive officers may not have the worst job in the world, but they certainly don't have the easiest. Managing thousands of employees, a contentious board of directors or a tough market, sometimes the most competent people have the cards stacked against them. Take Ron Johnson, for example. An extremely adept businessman, educated at Stanford and Harvard, he helped revolutionize Apple Inc.'s retail business after years of experience at other retail companies. Ousted yesterday as CEO of J.C. Penney Company, Inc., Johnson is being replaced by his predecessor, Myron Ullman.