Nice article at FoxNews (apparently linked in from Space.com) about the many unique and interrelated features of our shiny blue marble. First up is the location - at just the right orbit around our sun for the temperature to allow liquid water to form.
"It has liquid water, plate tectonics and an atmosphere that shelters it from the worst of the sun's rays. But many scientists agree our planet's most special feature might just be us. "It's the only planet we know of that has life," said Alan Boss, a planet formation theorist at the Carnegie Institution of Washington in Washington, D.C."
Liquid water allows for plate tectonics, the movement of the planet's land masses which allows for the carbon-silicate cycle (where the seafloor recycles carbon by subducting under the land masses) all of which also helps regulate the planet's temperature to keep the water largely in a liquid state. Plate tectonics may also help drive the rotation of the Earth's core, generating the magnetic field that protects the Earth and its atmosphere from the solar wind and high energy particles.
"Plate tectonics and water are inextricably linked. Not only does plate tectonics enable liquid water to exist by way of regulating the temperature, but many scientists have argued water enables plate tectonics to happen. "Without water the planet would be geologically dead," said Caltech's Mike Brown, discoverer of the newly reclassified "plutoid" object named Eris, which lies beyond Pluto in our solar system. "Water is what lubricates plate tectonics, which is what leads to the extreme difference between continents and seafloors, the large amount of earthquakes and volcanoes, fresh mountain-building. Venus has no water, no plate tectonics, no deep sea floor, no steep mountains, no continents, probably few earthquakes or volcanoes. A much less geologically interesting place!"
Another aspect is the Earth's size - large enough to hold an atmosphere, yet small enough to remain rocky and not grow into a gas giant. There is also the factor of the Moon, which drives the Earth's ocean tides and stabilizes the Earth's rotation, preventing the poles from wandering erratically, which could lead to massive climate shifts and prevent life from evolving. These tides may also have helped life along by giving an assist to life capable of living on land, which is the really the most unique aspect of our planet - life itself, and (so-called) intelligent life to boot. The examination of the Martian soils currently underway and future research on moons such as Titan and Europa are not likely to change this primary characteristic of Earth.