The lack of a decent magnetosphere on mars is probably the biggest issue we have to tackle if we are going to send people there. It would require some pretty significant infrastructure on mars to shield us and all of our life resources. We would never really live ON mars. We would live inside of containers that happen to be somewhere on/in mars.
http://science.nasa.gov/science-news/science-at-nasa/2001/as...
http://en.wikipedia.org/wiki/Terraforming_of_Mars#Magnetic_f...