Speaking as a doctor (I'm a DDS), I'd say that the two following characteristics are imperative:
1. Genuine interest in the well-being of your patients. One thing you'll quickly find is that patients generally don't inspire you to care about them. You, as a doctor, must have this intrinsic interest in their well-being, whether they're incredibly charming people or absolutely obnoxious, disrespectful individuals. Without this, you'll very quickly find your job to be a pain in the ass. And when it becomes that, the quality of your work suffers.
2. Genuine interest in your profession. Without that, you won't be compelled to learn. Medicine, dentistry, podiatry, optometry, and even chiropractic require ongoing learning. Although the formal education acquired from these doctoral programs put us far ahead of anyone else in health care (i.e. PA's, nurses, NP's, CRNA's, etc.), you'll learn far more on the job and through continuing education than you did in school and residency. The less your interest and appreciation you have for your profession, the less compelled you'll be to learn as much as you can and improve your skills as a clinician.
Interest in your patients, and interest in your profession as a science. The rest is just detail or icing on the cake.