What are the biggest hurdles for a mars mission?

i_ate_god writes: "Maybe I'm grossly under estimating the scope of a mission to mars, but in all honesty, what are the real hurdles that makes it so complicated? Technology wise, we've landed on the moon and have had two orbiting permanent space stations. We can take off and land on Earth and on the Moon. So whats so difficult about Mars? Go up, fly to Mars, land, go back up, fly to Earth, land. It's not as if we don't know how to do any of this. The only three hurdles I see is radiation, psychology, and how to land hundreds of tonnes of supplies properly. Are those three things really that awfully hard?"