What was so seemingly important about the differences between the North and South during the 1850's, besides them leading to the Civil War?
Of course, the differences between the two sections in the 1850s are most important (in retrospect) because they led to the Civil War. But at the time (before anyone knew the war would happen) they were important mainly for the Constitutional questions they raised about the United States and the relationship between the national government and the state governments.
The issues of the 1850s were largely centered around the issue of federalism (as it related to slavery, of course). The South continued to feel that the Union was a union of sovereign states. They felt that the Constitution had been ratified by the states and that the states had retained a great deal of sovereignty. By contrast, the North felt that the Constitution had been ratified by the people and that the people of the US were joined together, regardless of what state they lived in. This question about the interpretation of the Constitution would have seemed at the time to have major ramifications for the country. The question would have determined how strong US federalism would be; whether it would be more unitary with strong central control or more confederal with a great deal of state autonomy.