r/all has that randomness but it's mostly garbage, like mental fast-food.
Fact is, I can’t fully predict what I will find interesting so HN regularly offers me a good few dark horses that Reddit simply cannot.
Not long ago, there was a series on the history of bread. I’m sure there’s some kind of subreddit for this but I’d never have sought it out.
To that end, I think personal filters would be more useful than either tags or subreddits (even though HN already has ersatz subreddits in /ask, /show and /jobs.) The hide option is already a step in that direction, but it requires being reactive, whereas filters would be proactive. Give users a field in which they can list words or domains to either include or exclude (similar to the format of USENET killfiles) and have the feed sort based on that.
Then everyone gets what they want, and hopefully fewer people pollute the threads with complaints about Hacker News "turning into Reddit" because other people find things interesting and intellectually gratifying that they don't.