The Missouri Compromise was the first big indication that the whole country was changing its attitudes about slavery. In what way was the Compromise a positive action?
The Missouri Compromise in 1820 took place at a time when the United States was rapidly expanding in population, economy and westward movement. Very few people were thinking about or arguing for southern secession, but as slavery grew and cotton became more dominant in the southern economy, prominent southerners became more concerned about protecting and extending the institution in new territories.
As "The Great Compromiser", Senator Henry Clay helped to negotiate away some of that tension with the Missouri Compromise, allowing Maine to enter the Union as a free state and Missouri as a slave state, maintaining a North/South balance of sorts in Congress, and providing what seemed a common sense means of determining all future states' status based on the 36 degree 30' North latitude dividing line. So by avoiding a more tense situation in the US that may have fueled sectionalism, the Missouri Compromise was a positive agreement.
That being said, it solved very little. The Compromise merely postponed the inevitable political and later military conflict between northern and southern interests, and created conditions where slavery could grow even further to bring about that end.