When I switch to "no style" in Firefox, HN becomes pretty responsive. The only 2 problems I can see are that the textarea on top still has a fixed width and that the comments are not indented.
Mobile browsers should offer a custom style option, or at least a "no style" option like FF does.
Sidenote: Is it possible at all to have horizontal scrolling websites that Google consideres mobile friendly? Some websites are intended to have a horizontal scroll. What are they supposed to do?