Stress

You'll be in war zones, working night and day, getting very little sleep, doing very little in the way of personal hygiene, and constantly dealing with matters of life and death. Where do you go when it all gets to be too much? Do you just leave the hospital for a breather, maybe a week-long rest from all the war and disease?

 
Advice: don't make the decision to join MSF just after buying the cutest pup in the world. (Source)

No. The most stressful part of this job—and there are a lot of stressful parts—is you have to live with it. There's nowhere to retreat to. So you'd better find a method for dealing with your stress, and do it quickly. Meditate. Do yoga. Get therapy (which MSF will gladly pay for before and after your mission).

There's also family stress, as in stress caused by familial matters. Can you be away from your family for a year? Can they be without you for that long?