Thanks for the suggestions, I'll check them out.
I wouldn't be able to properly answer your question, but I'd say the art style really appeals to me, it can sometimes be a bit ugly and I'd like for DC to improve on some things, but it's an art style that is very easy to look at and feels natural. Other 2D games, like What a Legend for example, also have a great art style from an objective point of view but don't appeal to me all that much when it comes to actually finding it hot. Of course, animations play a big role as well.
But even apart from the actual nude/sex scenes, there's a lot of soul in Summertime's world because of the art and attention to detail that make it fun to play around in.
I also like the way StS sets up events and quest progression, can't elaborate on that due to not having experience in game design/writing but I think StS manages to play well with the fantasies of the player. I dunno, there just hasn't been a game that hits the same as Summertime Saga and evidently a lot of people agree with that sentiment because it's the most popular 2D adult game out there.