Only for the Hardcore - travel to Mars is a tough undertaking, check out this great article in the Register on just what it takes to be a successful interplanetary explorer with today’s state of technology. We’ve learned a lot since the Apollo Moon landings to make space travel safer and more comfortable, but in more ways than one a mission to Mars is still not for the feint of heart.
Mars One makes no bones about there being no hope at all of a return trip to Earth, and needs astronauts who are completely at ease with this situation. Assuming all this is fine, then the five key characteristics of a successful applicant are: resilience, adaptability, curiosity, the ability to trust and creativity/resourcefulness.
There are [also] good reasons why space agencies, credible or otherwise, place such a great emphasis on emotional and socio-psychological stability. Mars is a long way away (a minimum of 35 million miles or 54.6 million km and a maximum of about 401 million km), you’ll have to go there in a tin can with very little room to move, you cannot pick your shipmates, and there’s every likelihood that you will die on launch, en route, on arrival, or very shortly afterwards. It’s a lot to think about, and there’ll be plenty of time to dwell on it.