Skip this post unless you're using modern onenote on a tablet with a pen.
TL;DR - On windows 8 tablets with pens, pinch zoom in onenote is disabled. You can enable it via charms->settings->options->pinch zoom.
I love onenote, and really enjoy using modern onenote on my surface pro. The one feature that kept me from exclusively using modern onenote (as opposed to switching back to desktop onenote) is pinch zoom.
I was bemoaning lack of pinch zoom to my friend in onenote, who told me pinch zoom is implemented, just disabled on the surface pro, and other devices with pens. This disabling was done to avoid accidently drawing when you try to pinch zoom.
I've enabled pinch zoom and everything works as expected except for the case below which is easy to avoid.
If I use the finger menu with my finger, then try to pinch zoom, onenote will assume I want to draw with my finger, and draw a line instead of zooming. So long as I use my pen at least after using the finger menu with my finger, my next attempt to pinch zoom works.