Sure, here are some first impressions from someone who has no real experience with Jupyter, Observable, etc. :)
I'm really amazed at the seamless reading and editing experience. This is very well-made, I can see it must have been quite an effort to make it feel so effortless.
The ability for a code block to provide a "mini UI" is perfect. A reader can tweak controls and see immediate visual feedback, in the form of graphs, algorithms with variables..
One control I missed was a stop button - for the second example, I wanted to pause the generation of the 3D city/terrains, to continue reading.
I liked the dotted blue vertical lines to indicate "block focus", to see what it contains when I click edit.
While dragging a block, I had some difficulty crossing over another block which was long - in fact, I couldn't swap them, since dragging down didn't trigger the scroller to start. From what I know, this relates to the "center of gravity" during drag, whether the dragged block goes under another one. As for the scroller, maybe it's using a range that needs to be bigger, so that when I drag a block "below" the bottom of the screen, it starts scrolling.
Anyway, lots of fun and impressive work! This looks very useful for educational purposes. You're really hitting a sweet spot with the concept - (just iterating on this phrase to get my head around it), a social publishing platform for interactive content.
Great execution, and I'm looking forward to seeing it grow.
