This is a really interesting project.
It's also incomplete - but, in fairness, building a HN client that most people would consider "complete" is a massive undertaking (HN login, themes, customizable keyboard shortcuts, mouse support, pre-fetching, "read" status markers).
Instead, here's some feedback about the interesting/different parts of the design:
1. Having WASD, HJKL, and arrow-key navigation is nice - most tools commit to exactly one of those three.
2. I believe that not blurring the leftward (parent) column would be better than giving a toggle-able parent comment in the current column.
3. The keyboard shortcut + "Help" button in the corner is great - I regularly forget what bindings are available for a new tool that I'm in the first few minutes of using, and then often get bored and leave.
4. Placing the comment content all the way at the top of the box slightly encourages focus on that content, which is good - HN puts the username and posting age at the top instead, which is a distraction, and makes it slightly easier to think about users instead of content.
5. Lack of pagination is great - no more artificial second-page barrier.
6. You could use more of the screen space - for me, about a third is just empty, even when I zoom.
Less interesting stuff: there's some visual rendering bugs of italics and the "M" menu isn't keyboard-navigable (this isn't meant to be a feature request; I know you're focused on making the site keyboard-driven). Congrats on making a relatively resource-efficient and visual-space-efficient web app!