Not my area of study and others are almost certainly better able to answer this question than I am, but the basic answer is that China fell to the communists. I'm more familiar with the changes in US public perception, for instance over the debate about "Who Lost China?" But in those short five years you go from an Axis-Allies bi-polar world to a Communist-Capitalist bi-polar world. When the communists take over in China in 1949 it jumps to the other side of the ledger. Among other things, as a result of that change, "public opinion" matters quite a bit less than it would under a democratic system (not that Chang Kai-Shek was a democrat), but the US also substantively supported the other side.

The other, related issue is that the US, and MacArthur in particular, fundamentally underestimated the likelihood and impact of a Chinese intervention in the conflict. He was unable or unwilling to consider that a rapid advance to the Chinese border on the Yalu River would be viewed as a provocation to the Chinese, or that the Chinese communists would be so highly capable in their intervention. Were it not for rather extraordinary tenacity (to put it lightly), it seems likely to me that the entire 1st Marine Division (my grandfather among them, incidentally), would have been killed or captured at Chosin.

It's pop-military history, but Martin Russ' Breakout: The Chosin Reservoir Campaign, Korea 1950 is a great read.