Physical Danger
There's an extremely high level of personal danger associated with this job. MSF does its best to protect its doctors, but you'll still be riding around in a Jeep through active war zones, with nothing to protect you but a little white flag, your stylish MSF t-shirt, and eyes that say, "Please don't hurt me, I'm a doctor."
And if the war zone thing isn't enough, you'll also be exposed to all sorts of dangerous diseases that could kill you. Pretty easily. Make sure you watch over your shoulder, because the next strain of tuberculosis is probably just going to sprout legs and start chasing people.
Now, the MSF is sensitive to these sorts of things, and you can always turn down a mission if it's too far outside your danger comfort zone. And if you really want to go home once you're there, you can do that too. That said, you should definitely get used to the idea of a heightened level of danger before you sign up.