Mars' low surface pressure...

Mars' atmosphere is composed mostly of carbon dioxide, having a mean surface pressure of about 6 millibars. This should be compared with Earth's atmosphere, which has a surface pressure of about 1000 millibars.

The main effect of this low surface pressure is that the atmosphere can react far more quickly or efficiently to a given energy input than our atmosphere. For instance the combination of the low surface pressure (and martian surface properties) means that the atmosphere near the surface undergoes very large temperature swings each day (as seen by the Pathfinder ASI/Met instrument).

Mars' atmosphere experiences very strong thermal tides. These are global-scale atmospheric disturbances that are caused by solar heating of the atmosphere (they aren't forced by gravity like terrestrial ocean tides).

On Earth, atmospheric tides have very small (although measurable) amplitudes at the surface. On Mars, these tides have relatively large amplitudes, typically 0.5 millibars or so. Note that this is almost 10% of the surface pressure, and is a very large effect.

Autumn dust storm (MOC2-405)

The low surface pressure also makes it more difficult to lift dust from the surface of Mars during the initial phase of dust storms. Dust lifting is dependent on surface stress, a quantity akin to pressure, which is proportional to atmospheric density and hence surface pressure. Because this value is so low on Mars, winds have to be very strong (in excess of 40-50 mph) to lift dust into the atmosphere.

One interesting consequence of Mars' low surface pressure is that liquid water is unstable on the surface of Mars. The surface temperature can rise above 273K, (or 0C, or 32F, the melting point of water at a pressure of 1000 millibars), but since Mars' surface pressure is so low, any water ice on the surface simply sublimes into water vapour. For liquid water to exist on Mars therefore, you need to increase the surface pressure significantly. See the Mars FAQs page for more details.