The biggest obstacle is the low gravity and consequent low pressure. The Martian atmosphere would have to consist of almost-pure oxygen to be breathable.
Personally, I find Mars much less attractive than the moon for colonization. The only advantage that Mars has in my opinion is near-earth-like gravity, and that is both a blessing and a curse. The gravity well is what prevents our easy access to space. Leaving the moon's gravity is much easier than leaving either the Mars' or the earth's. The moon is closer, by far, of course. Both Mars and moon residents will need pressurized habitats and breathing equipment. The total lack of an atmosphere is in some ways an advantage, as there's no dust and grit flying around to gum up equipment. Also, solar arrays would better without an atmosphere in the way.
For the record, I'm a fan of building space habitats instead of colonizing planets. The question was first posed by G. K. O'Neill, who asked whether planets are really the best choice for settlements beyond the earth. I tend to agree with his assessment, that they are not. And we should instead work towards building Babylon 5 like space stations.