HN's minimalist UI designed to make elements unobtrusive and sometimes difficult to see, and new features being mentioned only in a single thread means most users will just not notice. It also doesn't help that HN is a conservative culture generally antagonistic towards any change to the layout, and people wouldn't want to notice those changes to begin with.
I've seen people not notice thread folding or the hide feature despite those also being "in plain sight." People don't know threads can span multiple pages - dang has to go out of his way to point that out because despite the UI being exactly the same as on the list pages, a "more" link at the bottom of the page, no one ever reads past the first thirty links on those pages to begin with.
I've seen people not notice thread folding or the hide feature despite those also being "in plain sight." People don't know threads can span multiple pages - dang has to go out of his way to point that out because despite the UI being exactly the same as on the list pages, a "more" link at the bottom of the page, no one ever reads past the first thirty links on those pages to begin with.