Turns out the sound we most connect with the whole system (that "Achievement Unlocked" beep) was an afterthought -- originally, Microsoft's team wasn't planning on having any notification of point scoring at all. The idea of achievements has evolved over time as well -- they did start out with the 5-50 and 1000 Gamerscore requirements, but since launch, devs have better learned how to use Achievements and how to customize them. Apparently there's a whole bunch of code and science behind the whole thing. Go figure! We always thought it was unicorn magic of some kind.