I will admit I watched School Days out of morbid curiosity, I did want to see exactly why it was that the majority of anime viewers regarded this project as a unmitigated disaster. School Days is claimed by some to be a parody of the harem romance genre of anime but speaking from my limited point of view I saw it more as an unintentional jab at Western style soap operas and teen targeted dramas than a parody of harem anime. That being said I go over a few things while trying to avoid spoilers, though by now thanks to video sharing and word of mouth the "surprise" ending has lost it's shock factor for most.
I'll cover the characters later on, since I feel that they are the main reason for the venom directed at this show, but the story is not without it's share of the blame. To sum it up as directly as possible it is the story of a boy who enlists the help of his friend who is a girl to win the favor of the girl of his dreams only to realize too late that the girl of his dreams was by his side all along. This is being very generous to this story but that was the "plot" I gathered from my viewing. The problem was the execution of that plot was consistantly derailed by unrealisticly disasterous choices made each episode by the characters resulting in a compounding train wreck that served more to leave a consistantly vile taste in the mouth of the viewer than advance the story. The majority of the plot developments after the first few episdoes required far too much suspension of disbelief to be taken seriously and coupled with the paper thin characters made each episode feel more like homework than entertainment.
I am by no means an expert of animation nor am I adept in picking out the various shortcuts used to save on time and budget. That being said there are scenes in the show that may have well been intended as a dramatic effect that come across as cheap and lazy. If you are enjoy being blown away by visuals and dynamic effects, look elsewhere, this show is exceptionally average compared to some of the other series I have watched.
I didn't really find any major issues with the sound or voice acting and since I do not mind listening to Japanese while reading subs so I didn't have any major feeling one way or another here.
It's really hard to talk about the characters without spoilers, so proceed with caution if you don't wish to know too many of the plot contrivances.
Makoto - The unrealisticly indecisive male lead who's actions defy any plausibility. Even granting that he may want to "have his cake and eat it too" he comes across as too much of a villian to be able to earn or keep the interest of the girls who supposedly are drawn to him.
Kotonoha - The shy introverted love intrest who's later personality flip makes no sense even given what she has gone through. From where she started to where she ends up requires too much suspension of disbelief to be tollerable.
Sekai - Probably the only semi-realistic character in this mess of a show, though that is taking into account the very low bar the other characters set. I could follow her motivations thoughout the series and while her grand finale was a reach, did seem to make sense.
Setsuna - Sekai's best friend. Tries to keep Makoto faithful to Sekai by sleeping with him, she also apparently has feelings for him since he said a nice thing to her one time, go figure.
Otome - Knew Makoto in a prior school where she apparently harbored feelings for him and despite apparently being fully aware of both his relationship status and habitual infidelity, you can figure out what happens next.
Taisuke - Makoto's best friend who is nothing more than forced slapstick filler. Decides he likes Kotonoha for some reason out of the blue, possibly her bust size, and she agrees to sleep with him despite supposedly being so obsessively in love with Makoto that she is losing her grip on reality.
Hikari - One of Sekai's friends who is significantly interested in Taisuke and therefore logically sleeps with Makoto.
Nanami - Another of Sekai's friends, athletic girl with short hair, that is all.
Kokoro - Kotonoha's pre-teen younger sister who presence in the story fills the viewer with the mortal dread that this show may actually "go there" especially given every other absurd and offensive thing that happens, luckily it does not, at least in the TV version of the show.
Kumi, Minami, Natsumi - Otome's circle of friends, before you ask, yes they do, all at once, no reason that makes any sense is given.
This may not end up being the worst anime I will ever watch but this is really really offensively bad. There is no character development, the story is contrived and Sekai the "shocker" ending is not enough to salvage this disaster. In the end this is a 12 episode adaptation of a pornographic visual novel, go watch paint dry, you'll have a better time.