"I Hear America Singing" can be seen as a celebration of work; to be more specific, it is a paean of praise for blue-collar work. White-collar workers are notable by their absence in Whitman's poem—a sign that, like many middle-class intellectuals, he has a somewhat Romantic conception of the real-life working conditions of ordinary people.
In romanticizing the work of mechanics, carpenters, masons, boatmen, and countless others, Whitman believes that he's celebrating the ordinary Joes of society, the unsung heroes of labor whose work is essential to the running of the American economy.
Yet in romanticizing America's ordinary workers, Whitman conveniently overlooks the appalling, often downright degrading conditions in which many such people were expected to work. Treating working people as heroes in the way that he does can all too easily provide an excuse for not dealing with the many challenges they have to face, such as securing shorter hours, benefits, and decent pay.
To be sure, this isn't Whitman's intention, but in listening to Americans “sing” he seems not to hear the plaintive cries of working men and women in distress. Given what we know of the treatment of working people in Whitman's day, perhaps his song is just a little too happy.