World War II Impact
With the entire world in shock after the devastation caused by the Second World War, the US in particular was reluctant to develop new land combat technology. While there was some progress, the main people in charge of the US military believed that nuclear weapons were the future, and so the army degenerated into, as one source puts it, a mob of civilians in uniforms, and the marines were also severely cut down in numbers. All of these men were armed with WWII weaponry, and at first had no useful anti-tank weapons or actual tanks. Fortunately for these troops, the Communists were also using outdated equipment, which meant that the US forces were not massacred outright.
Important Developments
The only important innovations in the Korean War were the use of Mobile Army Surgical Hospitals, the use of helicopters in logistics, and the use of swept-wing jet aircraft. When China entered the war, their air force was armed with new Soviet MiG-15 swept-wing jet fighters, which was able to briefly challenge US/UN air superiority until the US Air Force introduced its own swept-wing jet fighter, the F-86 Sabre. These swept-wing jet fighters were a major improvement over older straight-wing jets and were arguably the most important military development of the Korean War.