Motivation, training, and organization are all key here.
The KMT in the Second Sino-Japanese War was a broken mess struggling to rebuild itself following the defeats early in the war that cost them the best of their German-trained troops and their irreplaceable equipment. For the rest of the war, it was scrambling for men, equipment, money, and training, often having to rely on ill-trained, ill-equipped militia and on unreliable warlord allies to fight their battles. Whatever supplies they received were rare and gained through much squabbling with the Western Allies, due to arguments between Gen. Stilwell and Chiang Kai Shek. Plus, they were fighting for a total of twelve years, from 1937 to 1945; that's bound to be more exhausting than a mere two to three years in Korea. And yet, they still managed the occasional victory.
The People's Liberation Army in the Korean War is a very different beast. Formed from the volunteers and cadres of the Communist party, many were strong believers in Chinese Communist propaganda, and as such were more motivated to fight. If they weren't, there were bound to be a few political officers around to 'keep morale up'. While the KMT at least had the odd tank and the Chinese Communist "volunteers" were overwhelmingly infantry, they were much better trained and equipped per platoon, a result of the experience of the PLA in the final years of the Chinese Civil War They were trained to fight in both regular infantry formations and guerrilla warfare. They were fighting as members of a single army, so issues of communication, cooperation, and command were much less problematic than the KMT's in the Second Sino-Japanese War.
Plus, the victories of both wars have been seen differently. China's contributions in the Second World War were generally forgotten, both as a result of a Western focus on the European and Pacific theaters, and China becoming communist and thus "the enemy", not to be glorified but vilified. China in the Korean War fought the UN forces to a standstill, thus causing a shock among US and European troops who expected Chinese forces to be as weak as they were in WW2 (don't forget the whole racism thing was still big at this time).
Though while China "won" its goal in defending North Korea, it still failed in "destroying the imperialists". Mao said the West, especially America, was "a paper tiger", a weak enemy who would fold with one good hit. Turns out, the US turned the tide and forced a stalemate when the Chinese were certain (especially after their initial victories) that they were going to kick the Americans off the peninsula.