Well, I think the fears depend on where you are in the process.
First, is getting in.
Then it's can I handle the workload.
Then comes WTF have I done to my husband/wife and/or children?
And after you see what your loans are and think about what happens all four years you get oh, JEEZ THAT'S A LOT OF MONEY.
And you realize that you have to make choices. Will you study, be a gunner, get straight A's or will you actually spend time with your family? Guilt is a killer.
Then comes what OH CRAP. BOARDS.
Then rotations. I haven't learned ANYTHING the first two years.
Residency app season. What if I don't get in? What if they don't want me? What if I ask my spouse and family to move AGAIN?
Residency. Dang. I'm older than most of my attendings.
Residency. I stress even more about money and finances. Residency doesn't pay much. I'm like the guy on the lawnmower: I'm in debt up to my eyeballs. Nope, take that back. PAST my eyeballs. I can't even see the surface.
Nearing end of residency: light at the end of the tunnel! Looking at jobs.
I never really got the "making a mistake that could kill someone" stress. Perhaps I'm too used to it after doing the paramedic thing for so long. The mistake possibilities are a little different now, but the stress of it is relatively the same.
All that being said, the rewards are pretty darn great so far.