How did the movement West affect our nation?

This is a big question with many possible answers ranging from the impact of westward expansion on American politics to the influence of it on modern day pop culture.  Here is a look at some of the big picture implications of westward expansion.
1. The move West was a catalyst for the Civil War.  The Civil War likely would have happened sooner or later anyway but the addition of western states and territories escalated tensions.  Both abolitionists and pro-slavery groups wanted to bolster their numbers and eventually get more votes for their side in Congress by claiming the new Western areas as either free or slave territories.  While many compromises, including the Mason-Dixon line, attempted to keep things evenly balanced, there was always a new frontier to be contested.  While politicians on both sides might have been content if the opposing side could be limited geographically, both sides also could see the potential for the new western areas to tip the balance of power to one side or the other.  The Kansas-Nebraska Act in 1854 allowed settlers in new territories to make their own choice regarding slavery.  In response both sides sent people to Kansas hoping to influence policy one way or the other.  They elected two conflicting territorial governments and started a violent conflict known as "Bleeding Kansas".  While many western territories did not experience the same outright violence over slavery as Kansas did, the status of western territories in regards to slavery continued to be a question of great political weight and conflict.
2. The move of European settlers through the Western United States led to the genocide and forced removal of Native American tribes.  Conflicts between Native Americans and Europeans had been a constant since European colonization of the United States began.  In the West, however, the United States government adopted policies intent on the eradication of Native American peoples.  One of these policies was the wide-scale hunting of bison.  Buffalo served as a primary food source for Native Americans living on the Great Plains.  Without the herds of buffalo, many tribes were driven by starvation to accept resettlement on government reservations.  Through armed conflict, forced resettlement, and the introduction of new diseases not only did many Native Americans lose their lives, but also their culture, language, and religion.  In the 1960's and 70's the American Indian Movement or AIM staged a series of protests and occupied various locations in an attempt to force the government to address the continuing repercussions of European settlement.  In 2008 AIM staged a walk from San Francisco to Washington DC to again bring attention to these issues.  The actions taken against Native Americans during westward expansion are likely to continue to impact the country's future.
3. The resources of the West were the basis of the fortunes of many influential men throughout history.  Mining concerns, railroads, and the fur trade were the basis of wealth for many.  John Jacob Astor made much of his money in the fur trade and later established the New York Public Library.  The Astor family dominated New York social circles and the family name even inspired the naming of the neighborhood of Astoria.  George Hearst became wealthy through several mining ventures throughout the West.  He served as a senator in California and his son, William Randolph Hearst, would go on to revolutionize American journalism.  Leland Stanford earned his wealth as a merchant in the California gold rush and then later became president of the Southern Pacific Railroad.  Stanford would eventually establish Stanford University.  These are just a few examples of the economic impact of western resources.  Wealth accumulated in the West would have far-reaching consequences for the nation as a whole.