One big limitation that was glossed over in the presentation is radiation exposure. The only time it came up was the mention of a 'solar storm shelter'. The radiation exposure during the Martian transit would be much greater than the same time spent on the ISS, somewhere on the order of 200mSv [0] (the composition of this radiation also contains significantly greater proportions of heavy-ion radiation, which appears to be more damaging, so this may need to be adjusted upwards). According to the wonderful xkcd chart [1], this would be in the 'probably no radiation sickness, but certainly not good for you' territory.

I would love to know what their plans are, since shielding is heavy. [2] seems to suggest electromagnetic deflection as viable (which would be insanely cool, and could probably reuse SpaceX's cryogenics work for superconductors).

Also, a quick search didn't turn up much on the anisotropy of interplanetary radiation, but I wonder how much a reduction would be achieved by angling the crewless area of the ship towards the solar wind (which I think Musk had touched on in an earlier talk).

[0]: http://www.srl.caltech.edu/ACE/ASC/DATA/bibliography/ICRC200... [1]: https://xkcd.com/radiation/ [2]: https://engineering.dartmouth.edu/~d76205x/research/shieldin...

EDIT: Of course these sorts of talks are really exciting! This is just one more in a laundry list of crazy-cool engineering problems that have to be/are being solved.