1. Your attitude.
Can you sit at a keyboard for long hours typing what someone else has dictated? Can you refrain from the impulse to discuss with any other human being what you hear in those dictations, regardless of the integrity of that person, because it violates the patient's confidentiality? Can you take constructive criticism from others in order to advance your knowledge and abilities?
2. Your speed and accuracy in keyboarding (macros and expanders help here). Also a good working knowledge of grammar and punctuation are important.
3. Your ability to quickly find the correct answer(s).
A good hard copy library, good internet searching skills, good software are extremely important for your success but you need the knowledge of how to use them efficiently. A good school will teach you how to do this. Also, of help, are these boards after you have checked every other resource you have access to.