Although it's counterintuitive, in practice conformation is almost irrelevant. I've seen horses with perfect conformation that were then too lazy or too afraid to be any good, and others with obvious defects that were great performers.
Good thick cannon bones! There is so much landing force on their front legs with jumping so I like to make sure they have enough bone to handle it. 8" circumference at least is my preference. Otherwise something well put together with a good hind end. Mostly I look more at how they move than how they look.