Very good question. At least when I entered medical school in 1999 the following were true:
1) Many highly ranked medical schools are research universities. Therefore research as an undergraduate can look good...or even be necessary for certain schools.
2) A commitment to other human beings/public service, including of course some experience with the medical field. For example, I tutored a child with an autism spectrum disorder and wrote my application essay about this experience.
3) Being an "interesting" person. Of course one should not do things just to seem interesting. Nevertheless medical schools (unlike residency programs, mostly) are often very interested in things like artistic or athletic accomplishment, leadership activities, intellectual pursuits outside of medicine, etc. I can tell you schools were far more interested in my philosophy major than my biology major when I was applying, although excellent pre-med grades and MCAT scores are always a requirement.