What feelings were produced by the annexation of western lands in early mid 1800's?
Before the United States acquired the Louisiana Territory, the port of New Orleans and the mouth of the Mississippi River were closed to Americans. Americans who lived between the Appalachian Mountains and the Mississippi River had to ship their produce and import what they bought from overseas, through northeastern ports such as New York City, Philadelphia, and Baltimore. Thus northern and eastern merchants did not want the U. S. to own the Louisiana Territory because this would make it easier and cheaper for trade to go through New Orleans than through northeastern ports.
Almost from the start of government under the Constitution, there were businessmen and politicians in the North who wanted to use government to aid the growth of business. At the same time there were politicians and planters and yeomen in the South who did not want to pay the taxes that government aid to business would entail.
When Louisiana was purchased, Northern interests feared that more states would be created out of it that would be controlled by planters and Southern yeomen and their politicians. This would mean more senators and representatives who would oppose Northern interests in Congress. The Northern interests thought they would thus have too little power in the federal government. They were very upset about the Louisiana Purchase.
When the territory taken from Mexico was added to the U.S., Northern business and political interests tried to pass a law in Congress that would keep slaveowners out of the new states that would be created from the territory. This made Southerners very upset, because whether they owned slaves or not, they did not want to see Northerners achieve unchallanged control of the federal government.
I think the answer to this hinges a lot on whose feelings we are talking about and what constitutes the "early mid 1800s."
If the 1840s are included, then I would say that the positive feelings produced by annexation were patriotic in nature. People felt good that the United States was strong enough to take all that land away from Mexico. At the same time, negative feelings were produced because of the issue of slavery. The land taken during the Mexican American War upset the balance of slave and free states. This caused bad feelings between the North and the South.