My theory: IF they make the trip to Mars (and that's a pretty big IF) they have to worry about medical emergencies (what if someone needs an operation? on-the-site training? good luck with that.), air leaks, sandstorms messing with the equipment, meteors (on the trip there and after settling), "cabin fever" testing their sanity, and if those problems all get addressed: coping and living together with one another in those small cubicles for the rest of their lives.
It's a good thing that -
1. Humanity simply spawned into the 21st century with everything as is.
2. No human being as ever travelled to far off, dangerous lands or lived in isolation with little hope of seeing home again.
3. No human being has ever risked their lives in the name of discovery and exploration.
Okay, non-sarcastic answer - they'll be trained for 7 years which will include medical training, they'll have non-essential bits removed (like the appendix) to remove any future complications involving those from the equation, they'll be in constant contact with experts from Earth of all sorts of fields. Meteors, on the trip there? I don't know if you are aware of this, but meteors that travel around the solar system are predictable as they're caught by the gravity of the sun and planets. They have these things called telescopes and other equipment that have been watching the skies for many years now; they'd know what's in the flight path and could account for it. The risk of being hit by meteors or asteroids once they're living there would be way down on the list of things they should be concerned about, the chances of it are minor. Cabin fever - human beings live for long periods in isolation, like in the South Pole. All applicants would be psychologically tested, those that get through to training will be trained in different groups to see who works best with who, and their ability to handle isolation will be tested.
Quite frankly, (and I grant that it is all a big if, if Mars One is successful in getting people there) if 4 people get there, they'll have access to modern facilities, comforts, training and equipment that the great explorers of the past did not ever have.
Also.
There's a lot of stuff that's covered on the Mars One's website, they have an FAQ.
http://www.mars-one.com/faq/mission-to-mars/what-are-the-risks-of-dust-and-sand-on-mars
For the Conspiracy Theory nuts: if the governments are keeping "aliens" or UFO's secret, they aren't going to let these people off the ground. And if they do, they won't survive in space. Some kind of equipment malfunction will occur and they will either blow up or not leave at all. OR: the infamous "fake moon landing theory." They'll film the entire tv show on a sound stage or in the desert somewhere.
Why do we care what conspiracy theory nuts think?
I have no doubt in my mind that this WILL be the biggest astronomical disaster for the history books.
Are you guys born with psychic powers or is it something you attain?