The first task that a national curriculum fails to address is that of differing student ability. The large variation in general learning ability means that gifted students will be held back so that average students can keep up. This can easily lead to boredom, laziness and misbehavior. At the same time handicapped students will struggle to keep up unless the curiculum is significantly dumbed down. Combined with the incentives that evaluating teachers and schools by test scores pose, this is a recipe for making school more about daycare and less about learning.
A related critizism is that a national curriculum misses the opportunity not only to provide children with as much learning as each of them can handle, but to customize that learning to their interests and abilities. Some children are more gifted in general than others, but all children have strengths, weaknesses and interests that vary from their peers. While there is certainly some value to requiring all children to learn skills fundamental to modern life, children spend far more time in pre-collegiate schooling than is required for only that.
Lastly a national curriculum sacrifices the benefits of diverse skillsets in the population for the ease of having a single curriculum. For any given skill that's not part of the mandated courses, we may be foregoing that art's next great practitioner. Admitedly diversity in education leads to many students receiving sub-par education in certain areas, and we should definitely strive to makes sure that everyone is minimally proficient in important skills. However diversity also leads to stronger education in some areas. Every classroom becomes