This looks great. One minor problem I had while trying the demo:

  press [`] key (key to the left of #1) to hide/show subtasks

This doesn't work on a German keyboard layout where ^ is next to 1. Pressing the ` key doesn't work either.

Bookmarked. This reminds me of OneNote but brought online. Feature suggestion: annotations for your notes. I think students and 'paper-replacement' users might find something like that convenient combined with the regular note-taking tools.

I wouldn't recommend putting too much time into this. After thinking about it, I'm not sure how many of your users would find the feature useful. I have a tablet which includes a pen, but that makes me an outlier.

Awesome, especially the checklist feature (when it arrives).

Great app. I love its simplicity.