For the first time since September 11, 2001, companies will return to the World Trade Center for business. On Monday, publishing giant Condé Nast will become the first business to move its employees into One World Trade Center, the 104-story skyscraper built where the Twin Towers formerly stood. The $3.9 billion building was initially slated to open in 2006, but its construction was plagued with delays due to cost overruns and political squabbles. There are some financial concerns about the tallest building in the Western Hemisphere, since it has only leased about 60 percent of its space. Still, it’s a sign of revitalization for many New Yorkers. “It’s a beautiful thing,” said one of the construction workers who had worked at the World Trade Center site for 12 years. “People were depressed from before. Now, they are more happy that they see a positive change.’’