With Steam-integrated features, instead of relying on email for high score notices / takedowns, etc, we could use the steam-integrated messaging.
[So-and-so] just beat your high score in [song]!
Or having Steam report what songs your friends are playing, so you can right-click to play the same song as they are (if feasible).
Or, hell, a challenge mode: Challenge someone on your friends list to beat your score in a certain song.
Steam AudioSurf Achievements! (We could fill an entire thread with ideas there, I'm sure.)
Windows-shell support for right-clicking on an MP3 file to "Play This In AudioSurf..."
So, how about it, Dylan? Once you're done rolling around giggling gleefully in your growing mountain of proceeds from Steam orders, of course. (I wouldn't begrudge a man that for the world! *grin*)