Couple of thoughts I have for you from what I know about our class...
1) Avg GPA 3.7, Avg MCAT 11.1--make sure your stats are competitive with this
2) From what I have seen of interviewers and of my class makeup, Northwestern likes to hand pick 1 or 2 people from each school. What I mean is, there is not a school where an overwhelming number of people have gone (besides obviously NU undergrad). Like I am the only guy from UNC in my class even though many apply, there is only 2 UW-Madison students even though that school gives out a huge number of applications to NU med. So what I am trying to say is that what they don't tell you, that I feel from looking at my class, is that if you go to an uncommon school, it is to your advantage. The school really tries to pull a class together where as many people as possible went to different schools (like there aren't 20 people from Yale and 20 people from Harvard like some schools). You name it and we have one student who went there for undergrad--get what I mean? There are probably close to 100 schools represented here, maybe more.
3) One of the reasons I felt I got in, besides I had good stats but I am a white male which I think hurts me in the admissions office, was because I was CLEAR in my essay that NU was my #1 choice. It was indeed my #1 choice and I let them know that. So I feel a letter of intent, of which one of my essays basically was, can definetly help your chances.
If you have any other q's I'll try to help.