I have no qualms about my ability to perform well nor do I lack any confidence that I will succeed. I was just interested in what many people may already have had experience with...
Everybody comes in with a different set of strengths and weakness. Some students are more knowledgeable in certain basic science subjects than others at the beginning. At my medical school, there were students that had PhD's in biology and master's degrees in anatomy, for example. It definitely seemed like they had an early advantage, but at the pace that medical school runs, nearly everybody was on par, as time went on. It was a likely a slightly steeper learning curve for those w/o the background, but that can be an advantage, too. Less to unlearn, if the material is different, and you don't take anything for granted.
I also knew a few students in my school that had extensive clinical experience. Some were OB nurses, PA's, paramedics, etc. They did seem more familiar with the set up and understood the language of medicine better, but at the end, everybody seemed to be on pretty equal footing. Whatever it is they did in the past, they had to let it go and learn what it means to be a physician.
What it comes down to is that medical school has a sharp enough learning curve and goes fast enough that everybody, no matter what your background happens to be, is going to be trying to keep up. Whatever advantage there is from a given background gets quickly surpassed.
And I agree with the above comments from other posters. It's an individual game.