We investigate slope distributions in the northern hemisphere of Mars from topographic profiles collected by the Mars Orbiter Laser Altimeter. Analysis of the region from about 12°S to 82°N, over diverse geologic units, indicates that the range of regional-scale slopes is small, generally <3°. Surface smoothness is most distinctive in the vast northern hemisphere plains, where slopes are typically <1°. Amazonis Planitia is particularly remarkable in its smoothness, exhibiting an rms variation in topography of <2 m over a 100-km baseline. This relative smoothness is still present when compared with other sampled areas of the Martian northern hemisphere and with volcanically resurfaced terrains elsewhere in the solar system. Planetary surfaces of large areal extent that are most comparable to Amazonis in terms of rms elevation variation over long baselines are depositional in origin and include terrestrial oceanic abyssal plains and certain sedimentary basins. Slopes across the Valles Marineris canyon system show that the upper portion of the walls are significantly and consistently steeper than the lower walls, characteristic of extensive mass wasting. The observed long-runout is consistent with a high-energy collapsed flow. In the neighboring Noctis Labyrinthus canyons the duality between the upper and lower walls is reduced, and indicates a lower energy modificational history and/or greater cohesion of wall rock.