I feel like this discussion has digressed a bit...
I'd like to reply to Southern Gentlemen's post from my perspective as a Stanford medical student who has been doing interviews and secondary reviews for my school for the past three years.
First off, I know you're not specifically asking about Stanford, but there are a lot of similarities between how a lot of these schools approach admissions. Secondly, I can only speak for Stanford's process -- many of the aspects of the selection process is very school-specific. But I'll just tell you what I know.
For some reason, many people think that the primary thing that these schools look for is GPA and MCAT. That is simply not true. Stanford, just like Harvard, Yale, JHU, etc, can easily fill up their entire class with people who have 4.0 GPAs and 42 MCATs, but they choose not to, because it makes for a very boring class. So what purpose do your scores serve you? They are there to get your foot in the door. Stanford has a complex formula that they don't tell any of us that takes into account things like GPA, MCAT, what school you got these from, number of extracurriculars, etc. They make some arbitrary cut-off, and those who get above it get a secondary.
That's where I and the other file reviewers come in. We do review a number of secondaries of applicants who have MCATs of 30 or less, and they are not all URMs. Often those applicants have done some amazing extracurriculars or something along that line. But at that point, scores are not the primary thing we're evaluating. We do also take a look at rec letters, but I'll tell you a little secret: everyone's rec letters always look the same. Recommendations are SUPPOSED to be good, so good recommendations don't tell us anything except that the applicant is not bad.
We look for exceptional recommendations as well as negative recs. A negative rec raises a huge red flag. An exceptional rec (ie. "this student is one of the 5 best students I have worked with in my 20 years as a college professor") can do a huge amount for an applicant. A typical rec won't hurt or help an applicant.
As far as what we look for, it's pretty much the same for the secondaries as well as the interviews. We are looking for students who would fit in well at Stanford, and who we feel would significantly contribute to the medical school. That's kind of a general statement because the people who have been accepted here stand out in very different ways. As far as what Stanford emphasizes, leadership and initiative are #1. Things such as starting a club or organizing a community service activity, for example, are looked upon very highly. After leadership and initiative are things such as: research, creativity (may be expressed in many different ways), teaching, significant community service, significant clinical experience, potential interest in academic medicine.
Stanford is not really looking for applicants who have a little bit of everything, but they are looking for people who have really excelled in one or two things, and have some experience in a few of the other aspects. Also, you don't need to have every characteristic I listed above. I have many classmates who have no interest in academic medicine, several who had no real research experience when they applied, etc. Last but not least of these qualities, you HAVE to be a nice and mature person! This is true for every school. I can't tell you how many people have been rejected soley for the reason that they came across either in the secondary or interview as arrogant or immature. That's not to say you shouldn't promote yourself, because that's what this whole process is about. But you should definitely not carry the attitude that you're better than everyone else (believe it or not, I have come across a number of applicants who seemed to have that attitude!).
So many of the above posters gave you good advice. Do what you are interested in. If you are really interested in it, you will develop a much more significant involvement in that area, and have an overall better application in the future.
Sorry for such a long post!