Well, for me the meat of a game like this would be bonding with the characters. Character interaction and their reactions to the player's actions. New scenes, new interactions, and new art (the art is great!) unlocked as you progress with the characters.
Right now at least Yui seems like nothing more than a stat check. You increase affection by mechanically repeating the same few actions every day to increase it, and the end result is that some new actions will become accepted. Apart from that, there are no events or dialogue or reactions to convey the growth of the two's relationship. No surprise, no rejection, no regret, no embarrassment, no acceptance, no excitement. No real sense of interpersonal story arc or character growth. Apparently you can go from talking to her every day, to having a bath with her a few times, to fxxing her at night and her reaction is .. no reaction. No words whatsoever. Like all that just happened was entirely normal, as if it had been going on for a long time already?
I am sure that you are familiar with Monochrome Fantasy and the way the character responds to the player. She's not going to be automatically happy about everything you do, and she may certainly have a negative or surprised reaction to some things. That is why you can also sense that things change as you progress. Not only that, but new scenes become unlocked (e.g. in the bathroom or toilet). Monochrome Fantasy is pretty good, but you could certainly go a lot further with this idea, and I would love to see it.
I don't care too much about specific skills or items per se, but I see them as a feature that gives the game some variety and progression, something to look forward to (while also looking forward to new scenes and character interactions). Usually things that either unlock new abilities (whether in combat or in home; preferrably in home!) or that enhance the characters abilities (ability to earn more, rest better, cook better?) make for more interesting level progression than stat growth alone.
I don't actually know how many skills there are in the game right now; I think I saw about half a dozen, by which time the books became unreadable. I never got around to unlocking any of these skills, because I got bored with the apparent lack of character interaction. It felt as if I'd already seen everything there is to see at home, and the rest is just repetition of the same.