What I haven't seen are actual solutions for how to deal with the radiation. Yes, there is ongoing research from NASA, ESA, JAXA and others, and lots of interesting proposals like hydrogenated boron nanotubes, electromagnetic force fields, lithium shields etc. But no solutions even close to implementation.

Here's from an ESA blog post on the topic earlier this year:

> As it stands today, we can’t go to Mars due to radiation. It would be impossible to meet acceptable dose limits.

http://esa.int/Science_Exploration/Human_and_Robotic_Explora...

Are we going to send astronauts who have accepted they will likely die from the mission? Is getting a person quickly to mars worth such a suicide trip?

The final issue I believe will be a hurdle is the psychology. It's one thing sending people to the moon for a few days, or to the ISS for half a yesr where they can look out the window at Earth every day. But I'm not sure the human mind is going to stand up well to the type of extreme isolation a trip to Mars requires. It's certainly never been tested before.