I just started barrels on a 4 y/o.
She is VERY sensitive in the mouth and listens to my seat aids like a seasoned 15 y/o.
THAT is what I think makes a good barrel horse. A horse that is light in the mouth, and engages the hind end with the your seat cue.
There are plent of barrel racers the jerk on the mouth and spur, spur spur. But the truly good barrel racers, that actually make it far, are those who have light, responsive, level-headed horses to compete with.
I would rather have a good, responsive horse than win with spurring and yanking.