Generally, you need to realize that medicine is a conservative field. Your humor may break up 75% of the people, but you don't want to alienate the other 25%...
I really had a problem with this; my undergrad essays were great, and i was proud of them.
my med school essays were polished, but conservative.
now, i'm doing my residency applications, and the essay i'm writing is SOOOOO conservative. i can't even let my personality shine through. but honestly, that's the trend...
don't be too funny, man... wait to show it off in your interview.