That is fucking wondrous.
First of all, the impact of in-browser-music-making is not trivial or obvious. You can’t be blase about it.
But beyond that, what’s striking about this particular hack is how high-level it is. It pulls the end user up several levels from decisions like the selection of video clips or specification of harmonic progressions. The user is closer to the level of a music game like Guitar Hero, except that game play is open ended. You don’t score points by playing this game, you make music.
It would be cool to use a selection of these open-ended-music-games as the DJ for a party. Maybe there would be a person selecting and queuing up the games, so that there was some kind of playlist. The overall impression would be similar to parties where the music is coming from people doing Rock Band or Guitar Hero.
Know of anything similar out there? Instruments verging on games and vice versa?