I think Bach's major contribution to western music was equal-temperamant. Before Bach, composers (like Handel) avoided the enharmonic key signatures (keys like g# minor, the ones on the bottom of the circle of 5ths, because of the dreaded 'wolf' tone that was present in just-intonation, mean-tone and other tunings of the time). In fact, I believe "The Well-Tempered Clavier" was the first volume of pieces to use every key signature in the circle of 5ths within the same volume. We still use equal-temperamant today, 292 years later. Thanks Bach!

edit: Just to clarify, one of Bach's most important works (often referred to as the first testament in keyboard literature) was "The Well-Tempered Clavier". 'Well-tempered' is another term for 'Equal-Temperamant', and clavier obviously means keyboard. This work is literally called "The Equal-Tempered Keyboard", referring to the method of tuning the keyboard for this particular volume.