I think it's mainly in relation to the constraints of the game engine, and also that the game engine being flexible enough to enable the gimmicks. I haven't played DDLC and probably never will, but from what I've read about it, like games with similar core themes (not dating sim) it has some gimmicks that tend to stretch the capabilities of a closed-down game engine, sometimes requiring patches to the engine itself. In this case the game engine Renpy offers an extensive DSL that makes it easy to add story scenes, media and dialogues, but allows you to fall back to python to do some tricky things.

It does a lot of screwing with the interface and game data in ways most VNs do not.

(SPOILERS)

It breaks the fourth wall in unexpected, and deeply unsettling ways.

As a gamer you take for granted that, at any moment, you can simply exit. The UI is a trustworthy boundary between the imagined world of a horror game, and the comfort of reality. In DDLC, you don't even feel safe on the title screen.

Most ren'py games, even the very good ones, barely change the UI at all. Roadwarden doesn't look like a ren'py game at all... until you open the save menu, and then it looks exactly like a ren'py game. Having developed ren'py games, I can tell you why people avoid touching that part of the boilerplate code: it's the one part of ren'py where the abstractions aren't well thought out. It's very fragile. To me, that makes DDLC all the more impressive from a technical point of view. It warps and abuses the most rigid and uncooperative part of the engine, and to great narrative effect.