The PDF in one of Al's links above describes the problem with launch windows. If you're trying for a free ride, you can't be picky as to when you're launched. You'll be left in a geosynchronous transfer orbit (GTO) with a perigee of a few hundred miles and an apogee of about 23,200 miles. You may have to wait there for weeks or even months before the window to Mars opens. In addition, you'll likely need to adjust the plane of the orbit to match that of Mars.

All of this does make designing the vehicle more difficult. While in GTO, the vehicle will pass through the van Allen radiation belts twice per orbit. This might impact the amount of shielding required. Molyniya satellites have done this for decades, but they were designed for it. Other issues include thermal loads. If you design the vehicle for the colder environment around Mars, it might overheat around the Earth. One idea was a jettisonable heat shade. That might work, but it also introduces another failure mode. To keep the propellant requirements as low as possible, they're considering using lunar slingshots to fine tune the inclination before going to Mars. The Japanese proved this is possible.