Dave C,
you ask a very good question. There are specialties that are essentially "non" competitive where if you apply, you can essentially get in, provided that you are not a criminal or that you go to a US allopathic medical school.
Generally, these fields are family practice, peds, psychiatry, etc..
Sometimes, competitive fields, such as general surgery go through periods where they're not very competitive at all, like last year. Many good surgery programs did not fill their residency positions. This doesn't mean that so few people applied that they took everyone that applied and didn't fill. Generally, they took all "acceptable" candidates and still there were positions left over. So if you wanted to go into general surgery and applied to that program, as long as you were a decent person, even if you finished at the bottom of your medical school, you still have a reasonable chance of getting into that residency.
Radiology used to be that way a couple of years ago, but now it's become much more competitive.
But I do agree with Kimberi that doing nothing will generally guarantee nothing.