'Hidan no Aria' has just about every cliche harem elements in the book. The premise focusing on a special occupation to try to be unique, the characters strip and trip randomly for fan service, violent retribution for accidental infringement, archetype heroines, obvious tsundere stuttering, ordinary high school student with hidden special powers, protagonist is surrounded by girls who are crazy about him for no reason... I can go on and on.
The core premise is somewhat interesting. It's set in a school that trains mercenaries, and the students undergo various lethal training and get credits through actual missions. However, there are also a lot of stupid concepts going on, like all them being "butei" (combat detectives), the Hysteria Mode, and the fact that almost every character is descendent of a famous historical and fictional figures - Lupin the 3rd, Sherlock Holmes, Joan of Arc, Tooyama Kinshirou, Himiko, Count Dracula... it gets embarrassing to watch after a while.
The story consists of one improbable event after another. The character and villain's actions rarely make any sense. It gets so bad that a good guy would point the gun at the bad guy and say something like "there's no escape now", and the bad guy just runs without getting shot. I had no idea why this series needed to be about "detectives" as there were almost zero thinking going on. It's more like charge in first and see what happens next. It's also pretty absurd all the characters were carrying the gun and shoots around in the school campus, and of course, everything is bullet proof so no one gets hurt. The jokes get repetitive, with the protagonist getting into ecchi situation with random girls, and seen by one of the yandere/tsundere heroines and gets "kaza-ana".
Worst of all is the protagonist's "Hysteria Mode", which gets triggered from sexual arousal. This is the stupidest activation trigger I have ever heard, and serves only as a tool for fan service.
Art in this series is decent. The female characters are attractive, though extremely generic design. Character movement is very stiff and apparently low budget, but there are some nice slow motion action scenes that it's at least average quality in terms of animation.
The voicing for Aria is absolutely horrible, a stereotypical tsundere voice with zero distinction. Not only that, it gets seriously annoying after a while. Other characters' seiyuu were average at best. BGM is quite insignificant, and OP/ED were average.
There is nothing in 'Hidan no Aria' that sets it apart from hundreds of other harem series. It's obvious why a certain fansub group would trollsub a series like this, and in fact, the sound effects guys in this series started trolling in latter part of this series, like inserting sheep sounds at the mention of 「執事」, a word meaning "butler" but has similar pronunciation to Japanese word "sheep".
Admittedly, Riko's character was extremely hot both in design and personality, and ecchi in this series was decent. It's one of those series you watch to kill time, and see how bad it gets. B-But it's not like I enjoy watching this series or anything!
