Disclaimer: I play a lot of games, but dating sims are the ones I usually avoid. That said, I don't have a lot of field experience when it comes to Visual Novels. I might be incorrect or might overpraise!
Many might have thought that the fourth wall breaking and shock moments are all it takes to create a game like this. I've heard the same for Undertale, yet nobody have successfully imitate or one-upped it. There are way more that goes into the game than it seems, and those reinforce the experience into what it so now. This isn't a thesis so I'll only touch on what I thought is the most powerful tool being utilized.
Subverting player behavior. Notice that I didn't use the word 'expectations' like how everyone been using. Of course, bait-and-switch (not-so cutesy moe dating sim) is obviously deployed, but what I'm going to explain can only be executed in interactive mediums, that's right, video games.
Behavior subversion is a play on how players are expected to proceed with something. It is awfully similar to subverted trope but the player takes an active role and is expected to solve a video game problem but having their prior knowledge failed them. For example, the player failed to save Sayori's life, the common solution is either to restart or save-scum, right? Another brilliance is having act 2 (post sayo-nara) play out almost the same as act 1 yet different. Players can't help but retrace their 'happy thoughts' (act 1) and try to find comfort in familiarity, but is constantly betrayed by the widely contrasting outcomes and even the small, unsettling shifts (tilted background, out of tune music). Stack this on top of capitalizing on the concept of a computer application to aid its metanarrative, and playing the basic psychological horror cards right, it all comes to an unforgettable, unnerving experience.
This technique isn't easy to implement. It require substantial understanding of common gamer knowledge and behavior; game design knowledge to hint and help players to perform tasks outside of status quo; and guiding experience designed to properly lead players into accepting the notion instead of having it backfire. It is also rather niche, as it loses a lot of its value and power as more games do it. Luckily, the horror genre does have the advatage of tossing conventional designs aside for a good scare.
I have one small praise to add, for not holding back. The narrative is structured in a way that there is no going back. Once all hеll breaks loose (so early in the story, no less!), you are staying in hеll until the devil is dead. There's no jumping back and forth, no 'safe heaven'. Constant foreshadowing helps ease the transition. I really appreciate this bit.
Now of course the game is not without faults. Some have argued that this is all shock value and no substance; some would have said that player agency is non-existence. I agree to both points to some extend. According to the Fan Pack, it is designed to give nods to tropes but still maintain a level of authenticity. In the end the narrative kinda leads to nowhere and I think it helped the experience than harm it. If there is some sort of huge plotpoints and overarching story, it would ultimately distract the player from the finer details of the characters. Player decisions did matter, as it influences how the buildup unfolds, and the sequence of information revealed. Yes, in the end it all did not matter, but I have accepted that the journey is more important than the end. Although I do not mind seeing this expended, like new scenarios from toying with the character files mid-session.
Amazing game. Dan, you know your material and you are a genius. Collab with Toby Fox?