In direct response to crusade:
All characters hitboxes are normalized, which means they're all the same. From a purely technical standpoint, its not easier or harder for one race or another to land a jump becuase every characters physics are the same. Do gnomes run faster than tauren? no, but it looks like it when you have two people playing side by side. So do certain models jump better than others? Technicly no, It comes down to individual player skill and model animations.
Little things about certain character models can throw off your jump's positioning by messing with player perception.
For instance, Male undead lurch a little to the right, so you may be judging the jump a little imprecicely, which would be why people think that they're "not good", even though the jump is perfectly doable.
Same goes for tauren males. They're a very big model, it can be difficult to manage the camera angle and judge where their feet box actually is (where they would stand)
For the easiest time jumping, pick a small model with a stance that has the feet close together and a linear, smooth run animation. However, remember: everyone can do every jump, it just takes practice.