With the gentle caveat that this list of concerns feels fairly low effort — surely, with a little elbow grease, you can think of much more damning complaints! ;) — I'll try to respond in good faith:

1. The non-linearity of the code is really the whole point here. Reactivity means that you wan work on any little piece of the code at any time, and you don't have to reload the page, or re-evaluate the entire notebook. It closes the feedback loop, and speeds up the process of iterating on your ideas immensely.

2. We’ve added more keyboard shortcuts, so you can now "Shift-A" to select every cell, and then press "P" to pin every cell open at once (and then to close them again).

3. The full syntax is just the "Observable JavaScript" described in the OP. The Markdown tagged template literal is part of the standard library, and you can define your own that work just like it. In that sense, the syntax is extensible. For example, here’s a different "pl" tagged template literal that runs Prolog programs: https://observablehq.com/@mbostock/hello-tau-prolog

4. Every notebook is available for download and self-hosting. You can download the code as an ES Module, or as a tarball that you can install with npm, or load and render on a web page: https://observablehq.com/@observablehq/downloading-and-embed...