there are so many things that need to be figured out.
One of the most critical so far as i can see is understanding the long term implications of low gravity.
If thats not a deal breaker, then youd need to figure out how to protect from cosmic rays. I cant see a thriving world where everyone has to live under ground.
At least one bit of good news i read was that it wouldnt take too much to raise the pressure on Mars to allow walking around in shirt sleeves...more than a lifetime to achieve though.