A big rocket is probably the biggest obvious hurdle. What are other people doing to get us to mars that is better than musk's approach?
No, the biggest hurdle is having astronauts survive the journey (health and successful landing/departure of Mars). It would be over 3 years in space to round trip to Mars with current tech. We haven't had an astronaut in space continuously over ~1 year...the body suffers tremendously in a low gravity environment and you would have to handle SO many side effects and redundancies in life support etc.
Artificial gravity would address those issues. The big rocket (ie mass margin) also addresses life support redundancies (and allows you to use simpler, much more reliable methods like used on short duration spaceflights).
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No, the big rocket is the easy bit which has been done many times before.
Designing a mission involving 1000 days in space with no prospect of resupply is the hard bit. And it's the likes of ESA and NASA that have visibly been spending money on research and testing for that bit, not the very successful private launch vehicle supplier and LEO constellation operator.
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