Mainly the interaction (though the graphical design itself definitely isn't what you'd call modern). No notifications or gui animations, no thumbnails or header images, being as hard to navigate as possible, scroll position gets reset all the time, can't even reply to a comment while looking at the rest of the thread because everything is its own separate page, nothing has outlines so text has no clear separation, etc. In a nutshell it feels like a site made in the 90s entirely with HTML forms without a line of javascript.
I mean I do absolutely hate the new reddit redesign and almost exclusively use old.reddit, but the functionality there is still on another level when compared to HN.
