Often, the first draft is not the essay you originally envisioned. You might even discover that your ideas about your theme have changed as you have been writing. If you do not like the way certain sections are stated, re_arrange them. Once you are pleased with the body, make sure the conclusion and introduction still match it and complement the way you see your topic at this point. If not, rewrite them to fit the essay you did write and the way you see your topic now.
What separates the half that was accepted from the half that wasn't? One of the factors is the medical school personal statement. A strong medical school personal statement can really make a huge difference in getting accepted and rejected.