are two examples off the top of my head.
In my opinion, one of the biggest not-a-secret values is the looking into threads of others. It isn't a feature outright, but you just bookmark the comments of key people into a folder. For example, every now and then I check in on the comments of Putnam winner https://news.ycombinator.com/threads?id=cperciva
or maybe jump over to https://news.ycombinator.com/threads?id=dhouston the founder of Dropbox
Moot is always seems to make me chuckle: https://news.ycombinator.com/threads?id=moot
Of course there is https://news.ycombinator.com/threads?id=kn0thing too
...and you get the gist. Don't stalk it everyday as that'd be a waste of time, but from time to time it helps you catch up on something insightful you otherwise would have completely missed.
It's not unlocked by karma, but it's a relatively unknown feature.