The speed dependent zoom feature is awesome. Honestly, it's very nice! The one thing I would change about it is if it was more a continuous zoom, instead of the discrete/toggle zoom it is right now.
Basically, once a certain minimum scrolling speed is met, you start zooming out, and the faster you go, the more you zoom out up to some maximum zoom out (say whatever is set in the slider in the preferences). This would allow someone to subtly change the zoom and scrolling behavior at the same time.