There are many reasons given to explain why King David was considered so extraordinary, but the Midrash reveals that he was unique even before he was born. According to The Midrash, God showed Adam the entire future of humankind. Adam noticed one particularly bright soul that was full of potential but had no years of life attached to it. Adam offered to give this soul 70 years of his own life. Thus it was that David lived exactly 70 years, and that Adam lived 70 years short of a complete millennium.
David was born the eighth son of Jesse of the tribe of Judah. He was born under what seemed to be questionable circumstances (click here to read more). In addition, according to Talmud Sotah 10b, he had the unusual distinction of being born circumcised.
The Midrash also notes that King David's death was unusual. The Talmud, Shabbat 30a, relates that David was aware that he would die on Shabbat and wished to die on Sunday instead so that he could be buried without any delay. God told him that this was not possible, but David took matters into his own hands. He spent every Shabbat immersed in Torah study so that the Angel of Death would have no power over him. Not to be put off from his Divine mission, the Angel of Death caused a great noise in the orchard beside David's study. David continued to study as he went to see what the noise was, but paused momentarily when a step broke beneath him. In that moment, the Angel of Death completed his mission.