I think its good that HN and reddit are basically the same, or that all old forums were basically the same but with different color schemes. Homogeneity is a blessing for UX.
Honestly, HN and Reddit are almost as different as threaded discussion forums are possible to be, especially New Reddit with it's "click hundreds of times to unhide most of the text on this page" approach to threading. Reddit's overall design aesthetic is all about pictures and headings and sidebars, and even minor details like the up/down arrows look different and are placed in a different relative position. The only design element they've got in common is Verdana, and that simply because when the websites were launched you only had two widely-installed sans-serif fonts to choose from...
Honestly, HN and Reddit are almost as different as threaded discussion forums are possible to be, especially New Reddit with it's "click hundreds of times to unhide most of the text on this page" approach to threading. Reddit's overall design aesthetic is all about pictures and headings and sidebars, and even minor details like the up/down arrows look different and are placed in a different relative position. The only design element they've got in common is Verdana, and that simply because when the websites were launched you only had two widely-installed sans-serif fonts to choose from...