Crazy simple and thus misses a few rather basic features - like proper scaling (the icons for upvote/downvote and buttons for actions are too big for mobile, and also too small on big screens), and some basic late 2000s AJAX to avoid page reloads for basic actions such as adding/deleting a comment.
Never underestimate the ability for a company to chase customers which turns into feature requests which turns into complexity which turns into revenue which turns into profit and nice salaries.
HN is crazy simple. Even $500/m sounds a bit high imo.