Wow. That's all I could think; wow.
The characters and the writing. Seeing each of the characters react and cope with the existential dread from the end of the world made them feel much more alive. Hell, it even made ME reflect on myself as a person with how philosophically heavy it got.
I liked how abrupt the endings are when the THE happens. It genuinely felt like, "Oh shit, that really just happened.".
Overall, this is a 10/10 masterpiece. The writing gives you so much to think about. (This probably doesn't make any sense but I'll type it in anyways: this VN felt like pillow talk.)