The surface gravity on Mars is 38% of that on Earth. It is not known if this is enough to prevent the health problems associated with weightlessness.
Mars is much colder than Earth, with a mean surface temperature of −63 °C and a low of −140 °C. The lowest temperature ever recorded on Earth was −89.2 °C, in Antarctica.
There are no standing bodies of liquid water on the surface of Mars.
Because Mars is farther from the Sun, the amount of solar energy reaching the upper atmosphere (the solar constant) is less than half of what reaches the Earth's upper atmosphere or the Moon's surface. However, the solar energy that reaches the surface of Mars is not impeded by a thick atmosphere like on Earth.
Mars' orbit is more eccentric than Earth's, exacerbating temperature and solar constant variations.
The atmospheric pressure on Mars is ~6 mbar, far below the Armstrong Limit (61.8 mbar) at which people can survive without pressure suits. Since terraforming cannot be expected as a near-term solution, habitable structures on Mars would need to be constructed with pressure vessels similar to spacecraft, capable of containing a pressure between a third and a whole bar.
The Martian atmosphere consists mainly of carbon dioxide. Because of this, even with the reduced atmospheric pressure, the partial pressure of CO2 at the surface of Mars is some 52 times higher than on Earth. It also has significant levels of carbon monoxide.
Mars has a very weak magnetosphere, so it deflects solar winds poorly.
To Mars and back is a 2 year round trip, with a short stay on Mars.