Anything HN has had to implement, Reddit has to implement at a generalized, user-facing level, like mod tools.
Frankly, we underestimate how hard forums are, even simple ones. I learned this the hard way rebuilding a popular vBulletin forum into a bespoke forum system.
Every feature people expect from a forum turns into a fractal of smaller moving parts, consideration, and infinite polish. Letting users create and manage forums is an explosion of even more things that used to be simple private /admin tools.
I agree, most software is deceivingly simple from the outside. Once you start building it, you become more humble about the effort required to build anything moderately complex.
Its not that the mod tools are constantly being used, its that there's now potentially far more code complexity for those tools to even exist.
Since I've been here they've added vouching for banned users (and actually warning people beforehand) thread folding, Show HN, making the second chance pool public, thread hiding, the past page, various navigation links and the API. They've also been trying to get a mobile stylesheet to work. They've also mentioned making various changes for spam detection and performance. And the url now automatically loads a canonical version if it finds one, and the title is now automatically edited for brevity. And I've probably missed a few things.
And HN isn't a simple application by any means. Go look at the Arc Forum code - it isn't optimized for readability, or scalability or reliability, but joy - for the vibe of experimental academic hacking on a Lisp. It's made of brain farts. Hacker News is probably significantly more complex than that for being attached to a SV startup company and running 'business code' and whatnot.