I was watching a BBC documentary on the topic and it claimed that there were 50 000 American/U.N. casualties and half a million Chinese casualties. I looked it up but found no answers. (I found figures for Chinese deaths ranging from 100 000-500 000) What I did glean was that the Chinese military was heavily underequipped, even compared to the initial American invasion forces previously stationed in Japan, but were there other reasons? The documentary made it seem as if the Chinese strategy was of a much more potent nature than the Americans expected, yet the Chinese lost many more. Thank you!
Part of the answer is the difference in tactical doctrine adopted, U.N. forces favored greater firepower as a means to make up for the effectiveness of Chinese infiltration tactics. The technological superiority of UN forces in both air and artillery meant that the UN could strike Communists forces both on the battlefield but also far behind it as well whereas the Communist forces had no way of retaliating in kind to UN firepower until late 1952.
http://www.nj.gov/military/korea/effectiveness.pdf
(see page 11 of PDF for Korea)http://usacac.army.mil/cac2/cgsc/carl/download/csipubs/doughty.pdf