From a vocalist's point of view, selecting a key to best suit the singer's individual vocal range REMAINS paramount, but from a purely instrumental point of view, beyond the difficulty of playing in certain keys (different keys are more/less challenging to play on differing instruments), the sound of the same song will evoke a DIFFERENT mood. In general, sharp keys tend to sound brighter, and flat keys darker. Bach, Beethoven, and Mozart composed each of their works purposely in specific keys because of the specific mood it creates. People that are most sensitive to perceiving these key differences (all 12 keys) are those who born with perfect pitch.

Scott