Robert Zubrin figured out a plausible plan to put a colony on Mars, yet the article fails to mention Zubrin and his Mars (Semi-)Direct plans.
Colonizing Mars with a small team of experts is possible and within our technological and financial means to do so within a decade or so.
However, the interesting question to me is not "can/how can Mars be colonized" but rather why would people move to Mars in the first place.
"Freedom from Earthly oppression (perceived or imaginary)" cannot work as an argument: a Mars colony will basically have to be run like a prison with 1984-level surveillance. A single crazy person blowing up airlocks or sabotaging the water or food supply can deal a death blow to the whole colony, and the cost to rebuild/repopulate will be tremendous, therefore the entire investment will be watched very closely.
Secondly, the monetary cost of moving to Mars will be huge and will be even beyond the means of Earth multi-millionaires, reducing the number of possible colonists. Also, because of the extremely high cost of getting anything to Mars and a lack of native manufacturing, there can not be a co-operative ownership of Martian infrastructure, so in practise everything will be owned by some entity (corporation/government) operating from Earth.
These points might change and "freedom from Earthly oppression" might become a driver once the colonists become self-sufficient enough to build self-sustaining underground dwellings with locally produced tools.
Therefore I think a succesful colonization must have two things: firstly, some kind of business force to fuel the flow of colonists, and secondly colonists who are not idealists looking for a frontier world freedom utopia, since it will not be such, at least in the beginning.
As for the business force, I don't know what this could be, maybe mining or manufacturing or such.
So I don't think the colonization is gated by things like lack of gravity, surface radiation, or impossibility of terraforming, etc. The lack of a plausible business model which could profit from having people live on Mars is the main gating factor.