Only major problem I know of with terraforming Mars is how to give it a magnetic field. We'd have to somehow re-melt the interior of the planet. Otherwise, we could just put up with constant intense solar radiation, and atmosphere off-gassing into space. Maybe if we built a big fusion reactor in the middle of the planet...?
I recall estimating the power required to run an equatorial superconducting ring a few meters thick 1 km or so under the Mars surface with enough current to simulate Earth-like magnetic field. If I recall correctly, it would require about the current level of power generation on Earth to ramp it up over a century or so to the desired level. Then whatever is required to maintain it (mostly cooling the ring), which is very little. Of course, an accident interrupting the current flow would be an epic disaster.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.