Here is the important part.
So really I think these programs look at the whole applicant and also I think they may like the fact that you did an externship at the program and ya know, "audition" for the part.
Be well rounded. Be there (be attentive, be on time, be interested, extern or visit, etc.) Be interested, there are lots of ways to show this. Know the program. Why do you want to go there in particular? Know something about the people, who is there and what are their interests? Hope this points you in the right direction.
Grades are not that important (in my opinion). Nobody in my class had a 4.0, I think the top scores were around 3.8 but I'm not sure. I know mine wasn't. From what I was hearing, some other schools have 1/4-1/3 of the class with 4.0 GPAs. So what does that say?
Board scores are on a PASS/FAIL system. It only hurts if you fail or retest.
Extracurriculars can say a whole lot. The residency directors have been there. They know how to read between the lines. If you join a bunch of clubs just to have something to put on your CV it shows. If you have a few activities and are involved, that shows too. If you do things outside of school (in the community), that can be important too. (I.E. I was in the Reserves, and held part time jobs - which were medically related. I think it helped.) Bottom line is that they can figure out wether you might or might not be a good fit. In fact many of them will have a pretty good idea even before the interviews. That helps confirm their opinions.
Anyway, there are a few good pointers.
As far as this forum. Sorry, but I don't usually have time to visit often. There seems to be a lot of negative comments, a lot of which is unneeded. Don't worry about other people's opinions. Some of them do not deserve or need a response. Stay professional in your comments and responses, and they'll take care of themselves.
Take care.
Eric