In "Pioneers! O Pioneers!" America is awesome—and we mean that in the truest sense of the word. New territories to explore, new mountains to climb, new mines to dig, new lands to conquer. All this newness is what makes America, according to our speaker, so great. That, and the pioneering spirit of its inhabitants, who are always pushing westward.
Our speaker primarily admires America because it is a country in the process of being created, and its identity is still malleable and expanding. In a way what he's really praising is its potential, the fact that it has such capacity to become more and greater than it is.
Although our speaker uses America as his landscape, he is really only praising the creation and newness that pioneers represent. This poem could take place anywhere there are people venturing into new territory.