I, also, agree with Spyder. That question is a big one to answer.
Any horse CAN jump (unless there's a lameness issue, etc.), it's whether or not they enjoy it that makes them a good jumper. Of course, I look at the whole package when I'm looking for my next eventer or jumper. I want a horse that's BUILT the right way physically (i.e. Proportional, solid leg structure, short coupling, non-upright pasterns) and mentally (brave, quick learner, good work ethic, little bit of attitude).
"Always be yourself. Unless you can be a unicorn. Then, always be a unicorn."