What is the significance of Ravens?

The raven seems to play a part in every religion, as well as an integral part in cultures and mythology, why is this so? Is it becuase Raven's are so intelligent (ranked as tool-making intelligent) or is it becuase they are so widespread across the earth, or is it simply their seemingly close approximation to death? Also why is Christianity the only religion to have only "demonized" the raven, whereas all others seem to have good ravens and bad ravens?