I like equiniphile's answer; with a few caveats.
In order to jump correctly and well, a horse must move fairly close to its natural balance and outline, and feel absolutely free to use its head and neck in a balancing gesture.
A horse trained *incorrectly* in dressage, that can't carry itself in balance without support from the rider, will be difficult to train to jump. One that has been forced into a false frame and can not move between long and low, a relaxed frame, on the aids/training level frame wll be difficult to train and maybe even dangerous to jump.
A horse with a correct foundation that really moves from behind, rounds through its back and is supple and reponsive to the rider's aids will be easy and a joy to teach to jump.