Why is it cool?
* Every cell is live, reactive code that re-evaluates automatically whenever any of its inputs do. This includes awaiting promises and yielding new values from (potentially async) generators.
* A notebook can import and reuse any cell from any other published notebook.
* You have access to all of the computational goodies that the web platform gives you: graphics capabilities with <canvas> and <svg>, the GPU, the world's largest open source library with npm, and web APIs.
It's been interesting thinking about what it would mean to have a read/write medium for the web — where you're working with live values, and not in a "dead code" text editor. For just a teeny hint of some of the possibilities, check out Tom's autocompletion gif: https://twitter.com/tmcw/status/966073331207233536