The tutorial notebook is full of individual notes with multiple 'cells'. When creating my own note, it's not immediately obvious how I achieve this. Hitting ENTER just creates a new paragraph/line within the same cell. I've discovered, through trial and error, that hitting SHIFT+ENTER creates a new cell, but this should maybe be more easily discoverable.
Some of the UI elements are a bit out-of-place. I'm not sure if it's just because they're not native controls; I'm still running on Mavericks and it's quite possible these are El Capitan-style controls. Still, it's a tiny bit disconcerting.
On a more positive note, I think this is a wonderful concept which can potentially inform future tools for 'code management' (for want of a better term). I think there are plenty of ways in which the integration of code and other content can improve our discipline, so any work in this area is very interesting.