I'm not sure exactly what they look for in racers...pretty sure it's something like a deep girth to make room for the lungs and heart for the heavy work load, and a powerful shoulder and hip. Don't take my word on that though lol
But as far as jumping and other sport horses go, I feel that if a horse is well conformed or as close to "textbook" as possible that horse has the potential to perform well in any event (jumping, eventing, dressage, hunters etc). But there are some traits or tweeks in the conformation are better for some disiplines than others. Look at this link and you'll see what I mean: Book Excerpt: Which Thoroughbred Best Fits My Needs?