This is all just my personal opinion, and that is worth very little lol buuuuut ....
The horse has to have GREAT ground manner and be able to read your body language. I think its important for the horse to know what its supose to do, before you ask it to ....that may be bad, BUT it can be good too! Also, a horse that responds well to pressure on the halter, this to me is verrrry important. They should instantly react to just the slightest pressure applied to the halter noseband or the poll.
Also IMO, i think its important (especially for hunters!) to almost 'neck rein' over, my horses know when i lay the lead on their neck, they move off of it. My gelding, LTD's White Chief, if he even senses the slightest move in the lead towards his neck he moves off of it. It is such a handy concept to teach your jumping horses.
Also, i like a horse that will trot at the end of the lead, i dont like a horse that is right up on me, now i wouldnt ask them to do that at a show, but i dont want a hose pushing me around a jump.
Physically, i think a horse with a nice hip would be able to propel themselves up over the jump easier then a horse with a cruddy weak hip, which could make their form better too. A horse that is conditioned and in shape can jump nicer then a horse that is overweight (or even underweight) and no in top physical condition.
Most of my horses have a little extra spark to them, however, my best jumping horse though is my calmest horse, Chief.