My only complaint is that they never made any episodes that featured the teachers, since he also involved them in his revenge from what I remember of the game/visual novel.
Also, from what I know of, is this the only hentai anime that features time stop as the primary factor for the sex scenes. Though I do love that all four episodes just go directly into the sex scenes, I sort of hoped to learn more about the watch and the woman who gave the protagonist his ability to take his revenge. Seeing the bastard father's face would've also been a great bonus.
coming from someone who has no knowledge of the game/visual novel, i do agree it should have shown the teachers more, especially that teacher that appeared out of nowhere in the last episode. I don't know any other hentai with time stop which is weird.
As for the whys and hows, this IS hentai with very minimal plot. nobody is expecting a deep driven story with interesting background characters.