One obvious difference between Earth and other planets in the Solar System is that it's 70% covered in water. There are at least five major (and very different) theories proposing the origins of such a large volume of water (estimated at 1.3 billion cubic kilometres).
“The question of the origin of water on Earth, or the question of why there is clearly more water on the Earth than on the other planets of the Solar System, has not been clarified.” source Wikipedia
Also see: New Scientist (Nov.2010)
Ice-rich comets or asteroids from farther out in the solar system could have supplied it, but that raises a further problem. Comets are richer in deuterium, a stable heavy isotope of hydrogen, than Earth’s oceans. And asteroids should have brought more platinum and other rare elements than have been found. These mismatches are difficult to explain if most of Earth’s water came from impacts.
Now, it seems that water may after all have been present in Earth’s building blocks. Simulations by Nora de Leeuw of University College London and colleagues suggest that the dust grains from which Earth formed had such a tenacious grip on water that they could have held onto the molecules despite the high temperatures.