Not only did he compose a lot of wonderful music but he helped standardize piano tuning. Thus to some extent what everyone in the world hears as "in tune" (excpet for some non-western musical traditions, like North Indian) is a reflection of his work. That's a lot of impact.
My favorite classical composer, bar none. His chorales and fugues are absolutely brilliant. Especially when performed by good choirs. Happy Birthday, J.S. You sent the right tone for all future composers.
Compare his masterpieces with the garbage which we are subject to now blasting on many FM stations, and it makes you want to throw up.