1) Debussy's music is impressionistic in that through it he strived to convey a sense of ambiguity rather than realism, the shadowy impression of rather than a literal representation of reality.
2) Debussy is considered one of the fathers of modernist music, and is loved by (some) postmodern listeners due to his deconstructionist tendencies. (He broke the boundaries of classical timing, i.e. you cannot not play his music with a metronome.)
3) His music is meant to illicit deep emotion in the listener, bringing in colorful variation and vast free-flowing musical landscapes.
4) Debussy was French.
And yet, I somehow love Debussy. These reasons (apart from the inherent Frenchness) have provoked a search for a solid philosophy of music. Onward.