Happiness is sometimes brought by a rich pile of colorful images into one's mind.
This song is indeed colorful as Sufjan and his band play a rich variety of instruments (Sufjan is a real multi-instrumentalist, check the album info) throughout it. I recommended this song, maybe 3 years ago, to a friend in US and described it as a happy song, which, I thought, would be what he needed at that time:). I hoped, whenever he'd hear it there would be a little, childish smile on his face. This is what has happened to me whenever I've listened to it.
The symphonical use of instruments creates a joyful melody: a basis for the expression of ups and downs in Sufjan's state of feeling.
Well, lyrics.. for the first part, i can say: World's Columbian Exposition 1893, consumerism, art as a tool for capitalism, etc.. For the second, Sufjan questions himself about whether he can sincerely express his feelings in his lines or not.
When the gost of Carl Sandburg asks him if he's writing from the heart, he doesn't even care if Sufjan is 'the Devil'... he just asks: "is it from the heart?"