My humble observations based upon some experience of working with US Army during training:
1. Average intelligence level of a grunt vs. a typical soldier from this part of the world is higher.
2. Adoption to complex tactical equipment is higher in a typical US soldier.
3. Training levels are much higher due to higher budgets as compared to what we have access to in terms finances.
4. Training is more versatile due to global reach.
5. A system of learning and putting things down in writing, at all levels, is prevalent in US Armed forces that results in more practical and upto date Field Instruction Manuals and passing on field experience is a continueous process.
6. Going by the book is prevlent vs. us cutting corners results in better quality of training.
7. Seriousness of the officer cadre towards training and understanding of their benifits results in a more robust training regimen and hence a better trained soldier.
Overall, field experience, acutal combat in varied environments and against array of "enemies," its after action analysis, incorporating that experience into instruction manuals and exercises, and institutionlize the learninig cycle coupled with larger training budgets has made US Army one of the most trained armies of the world.