First off would be survivability. We have to know that humans can survive the journey to and from Mars. I personally believe that the technology exists to facilitate a trip to and from Mars, but I don't believe it's been tested in space (artificial magnetic field).
Next would be survivability (again). We have to know that we can bring people back, safely (unless they volunteer for a suicide mission). This means we have to demonstrate that we can launch and land some sort of functional refueling station on Mars.
Next, we have to demonstrate that we can launch and land a return vehicle that can remotely refuel itself from said refueling station and return to Earth.
We also need to launch and land some type of habitat for our astronauts near the return vehicle and verify it's sustainability. At the very least, we need to source liquid water. We also will probably need to demonstrate the ability to produce oxygen/grow food.
All of this needs to be tested, verified, retested and reverified several times over before any human astronaut straps down in a ship heading towards Mars.
-Wolf sends