I wrote a brief blog post on some of the major challenges to get this going. Musk's hesitancy to a deadline is great. :) It makes sense, hard to predict the future.
http://engineersf.com/12-reasons-spacex-wont-fly-a-manned-mi...
There's a ton of cool problems lurking around the corner. I hope humanity backs the public part of this public-private partnership.
1. Cargo Capacity — Scaling is Hard.
2. Proper Maintenance — Accessibility is Important
3. MTBF Expectations are too high.
4. Jet Fuel is Corrosive and Methalox engines are a tough
design proposition.
5. Cosmic Radiation — Impedes human interoperability.
6. Solar Panels —Mars Dust Storms impede sunlight.
7. Living Module — 7 month duration for a living module.
8. Microbial Realities — We rely on microbes to live.
9. Parachute Design — Size vs. Thrust vs. Jettisoning Fuel
10. Electronic Protection — Shielding is Resource/Weight
Intensive.
11. Eye Sight — Your ability to see diminishes in space and we don’t quite know how this works fully. (This one is huge)
12. Muscle Loss- you lose muscle mass as you stay longer in space.
To give some context around how difficult it is to build mega engineering projects in the hey day of innovation, just think about Steel. There's over 3K different types of steel and 70%+ were invented in the last 20 years.
Timing, sequence, funding, and focus are going to be such a huge part of this.
5,8,11,12 are really tough. I think the other ones are solvable in some way right now, but will take some configuration/tinkering/experimentation for sure. Plenty of engineers are motivated to work on this kind of thing though, so that's a good signal.