GPA and MCAT scores definitely seem to be the biggest factor in getting into med school. They are the easiest things for medical schools to measure and compare from applicant to applicant. GPA is a little more elusive as the quality of the undergrad college is a factor as well - I honestly don't know how admissions committees factor in the undergrad college.

The "extra" experiences are of unclear importance in my mind. I did some volunteering with the Red Cross at the hospital associated with my undergrad college. I honestly did not find this particular experience all that helpful though - I essentially did various duties on an inpatient surgical floor that the nurses wanted me to do for them. Aside from spending time in the hospital, I didn't really learn much. I'm guessing experiences with this type of volunteer work can vary dramatically based on who is supervising your volunteer experience.

I did get involved in research early on as an undergrad. Depending on the college you attend, exposure to research programs will vary significantly. While places talk about research a bit when applying to medical schools, I'm not sure how much this really adds or detracts from an application. I think the best way to find out about research activities is to go to the department office and ask them if they have a list of people's research activities. Usually they have it all outlined in their graduate school information they send out to applicants. You can go through there and see if any of the programs look interesting. If so, I'd call the professor doing the research and explain your situation and see if they have anything you can help out with.

This is exactly what I did - as I found within the chemistry department some interesting looking research on psychopharmacology of addicition in rat models. The professor was great with meeting with me and getting me involved right from the start. He assigned me a post-doc student to work with and they started me with the most basic things. First, I was given a few of the labs recent publications (which were WAY over my head - at first). The post-doc student went through them with me when he had some time. He also had me doing "grunt" work - such as going to the library to pull articles. He was great with how he presented it - because he pointed out that this way, I could read them first before he even saw them. I actually think I did pick up a lot of knowledge on the topics this way. As I went through college, I got to the point where I did an honor's project with my own research study, where I wrote up my results to present for an honor's thesis. It wasn't advanced enough to get published in a journal as it was, but I believe the research was incorporated into some graduate student's projects that did later get published. I had a lot of fun with this stuff in college, and I continued on doing very similar research as a medical student.

As for other extracurriculars - I think it is just a matter of unique experiences you can bring to the table. These activities don't have to be medical related either - I think just bringing a diversity of experiences to the medical school is of value. I don't think this is something you can go out of your way to do - you either have done it or not. I personally don't think I was too strong in this area, but it was clear several other medical students in my class were strong in this area - many who had careers before they went back to medical school.