I've heard several scouts say that what they look for is often somewhat simpler than many people realize. They look for:
Guys who can hit fastballs
Guys who can throw fastballs
That might sound like a gross over-simplification, but think about this.....When was the last time you heard of a guy getting drafted because he could really crush a curveball? Or the last time you heard of a guy getting drafted because he had a nasty changeup? Many things are evaluated, but sometimes choices are made based on some fairly simple and fundamental abilities that can't always be taught.