I don't even know where to start...
If I were to describe the writing and character design of Grisaia in one word, I would go for "inconsistent". Give me two words, and I'd say "wasted potential". And it's really wasted in an especially dumb way, because Grisaia is basically an Eroge where the writers have put a lot of effort into story and characters, but never shifted the focus away from the H content, which ruins a lot.
The amount of dirty talk that the heroines produce on a daily basis would put any group of pubescent boys to shame. I can't remember them ever talking about kissing, but I do recall a lot of instances where they're hinting at how badly they want their cherry popped. Either they're always watching porn whenever they're off-screen, or this Yuuji guy produces some ridiculously powerful pheromones. That, coupled with a lot of pointless fanservice makes the characters less believable and breaks immersion. Sometimes, as much as 1/3 of a route is spent on fanservice and H-scenes. Sometimes, the transition to H-scenes is so horrible, it makes you want to facepalm for hours: "Blew up my school. Time to have sex". Sometimes, the events are so disturbing and cringeworthy that you seriously have to ask yourself if you really want to continue playing this game. I mean, come the heck on, a heroine that looks and acts like a grade schooler AND calls the protagonist "papa" should never, ever have H-scenes with him!! To express it with Kanon's words:
So the anime actually captured the core of Grisaia pretty well: Fanservice has the highest priority.
To me, Grisaia looks like an overly optimistic fantasy of a perverted old dude that begins with "If only I was young again". Kazami Yuuji is an extremely unrealistic protagonist. Not only is he ridiculously overpowered and has the 'perfect' personality, he talks like someone three times his age. For example, he knows about ALL the things that make women annoying and how to deal with them. It goes beyond mere book-wisdom or stuff that someone else might have taught him. The way it is presented, it sounds like genuine life experience that he couldn't possibly have attained considering his age and backstory. Of course, he is super popular not only with the heroines but literally every female he ever encountered. His only shortcoming is that he is that he lacks common sense, but hey, so do all the heroines.
So what IS good about Grisaia? Well, most of the characters (the ones that weren't produced by wishful thinking) and some routes (the ones that aren't centered around fanservice and utterly pointless over-the-top brutality). Bad writing and pacing issues (in the VN) has been mentioned often enough on this topic, so I won't elaborate on that.
This might sound weird at this point, but I do consider Grisaia as a whole quite enjoyable. But it could have been so, so much more enjoyable ...