well.. It depends on the point of view i guess.. From Yi clan's point view, it was the best thing ever happened to them, however, from the perspective of a person who loves Korean history(like me
), it was a bad bad thing..
Because of Yi Sungye's high regard for neo-confucian ideals and the idea of "Sadeh"(China is the big brother of Korea) several things happened. Most of them being negative.
Under neo-confucian laws, only litrature, farming and filial piety were considered valuable. On the other hand anything practical were considered to be lowly, so professionals in science, economy, warfare, manufacturing and craftsmanship were all given second or third class status. Here are the details of what happened:
1. Under Yi clan's rule in Joseon, craftsmanship was looked down upon as lowly and useless. As a result, fine arts, intricate detailing of jewelry and many other crafts were discouraged and the quality went down greatly. Under Joseon, the line of Koryo, Beckjeh and Shila's incredible craftsmanship ended.
2. According to the neo-confucian ideals, the practice of martial arts were discouraged and the warrior class was looked down upon. Well, that doesn't sound like a good idea when you got neighbours like China, Japan, Mongolia and Manchuria, does it?
3. According to the neo-confucian ideals, farming was regarded as the root of Korean economy. Because of this .. Despite the fact that almost half of the country's tax income was through commerce and trading, Joseon discouraged commerce and considered merchants as lowly. What a way to ruin your economy..
4. Because of Sadeh(idea of China as big brother), Joseon effectively ended the official trade partnership with the rest of the world, therefore earning the nickname "Hermit Kingdom". Where as Koryo had trading relationship with China, Japan, India, Vietnam, Arabia, Persia and many other nations, Joseon only dealt with China and Japan. Why? China told Joseon to do so, and you do what your big brother tell you to do!
5. So why did China make Joseon to do that? Well... Korea always posed a threat to northern China, so it was important for China to break Korea's hands and knees when Yi Sungye volunteered to pledge loyalty to China. There were three ways to weaken Korea's strength and China did it all.. First, isolate Korea: less nations to trade with = little econmy. Second, request payments for "protection" = little economy. Third, request thousands of horses and restrict gun powders = little cavarly and weak infantry.
These are a few of many negative changes that were made during the change over from Koryo to Joseon. I can't say that there weren't any good changes (like advances in litrature), but those are very minor compared to major negatives that were accumulated over time and ultimately caused Korea's downfall in the 20th century.