I was thinking a few more menu items in the otherwise unchanged top row of HN - not making categories to file stuff into. There's ask and jobs, the rest could be up to each individual user, without having any effect on anyone else.

Reddit doesn't allow one to fold away particular threads, or favourite others - for me there is a difference between upvoting something and filing it away for later reference/response... oh, and maybe also a link showing just the new comments of all the discussions you favourited. While I'm dreaming, even let us give our own tags to discussion threads.

Maybe you could do something with tags.

Interestingly, there's already an implicit HN 'subreddit' (ask) and a couple of implicit tags 'show hn, ask hn, ask pg.' Even the who's hiring threads could be argued to be a "board" in anything but name.