Brynhildr in the Darkness follows Ryouta Murakami who had spent most of his childhood looking at stars with Kuroneko. One day, Kuroneko decides to show Ryouta the aliens she had found but on the way, an accident occur which left Murakami alive and Kuroneko dead.
Ryouta has been unable to forgive himself and dedicate his studies into discovering the existence of aliens. However, one day a new transfer student by the name Neko Kuroha appeared in his class who happen to look just like Kuroneko.
The First Opening - I just really like it and now one of my favorite opening. The second opening not as much.
Competent Main Character - This is a harem, even if it's darker and more of a sci-fi thriller, so it's not surprising that the show revolves around a single guy. In most harem, the main guy is often clumsy or clueless but not Brynhildr in the Darkness, Ryouta is one of the competent guys in the harem genre. So, it's not to see a change of pace.
The Blood And Gore - Normally, I'm not one for blood and gore but in this case, I find them beautifully animated.
The Animation - The animation is a hit or miss. At times, the animations are beautifully done while sometimes it feels a little lacking in detail especially when the scene zooms out on the characters.
Soundtrack - Aside from the awesome opening, most of the soundtrack for the show is forgetful.
Poor Adaptation - Despite the initial episodes getting me hook, the anime just falls completely flat in the latter half of the show. Why and how does it fall flat? The whole show was just a rush job and skipped over a lot of essential details.
If you're wondering how big was this rush job in the anime, well about a little over 100 chapters condense into 13 episode. And these are not slow nor filler chapters that is common with Bleach and Naruto. The first 4 episodes cover about 12 chapters. So about 3 chapters per episode but around episode 5 onward, the show starts making leaps while glossing over conversations, major events, downplay several characters' role, and even just ignoring some of the characters who's interaction give you further understanding of other characters. The last 2-3 episodes alone covered about 60 chapters.
This level of uneven compression really threw off the pacing.
Generic Characters - The characters devolve into something completely bland and one dimensional. The show doesn't give details or insight as to the motive nor their thoughts to many of the witches. Some of the major characters are downplay to an insignificant background role.
Also, this in turn makes the witches seem really dependent on Ryouta when they are capable of so much more.
[SPOILERS FOR BOTH ANIME AND MANGA]
I personally dislike Kazumi but for all the reasons I dislike her for, none of them happen in the show or even get a mention. She's the most disingenuous one out of all the witches. Most of her goals, and motives are far more for her personal gain/image and she often rationalize her selfish choices. Example, a whole even where Vingulf tries to capture #1107, Kazumi did half-ass try and wished to get Neko killed so she can be with Ryouta. Of course, she figure that Ryouta has a plan but she still intentionally put Neko in harm's way (even with the premonition of Neko's death by her choice) in small hopes that she may not make it. Then there is the fact, Kazumi hides information from Ryouta because it will strengthen Ryouta and Neko's relationship.
As for Valkyrie, the manga shows shows her progressively losing the little sanity she had left as well as how much Neko has mature in comparison to her. The anime, the Valkyrie just seems like a hollow shell of a person.
Also, poor Kana, they made her so insignificant compare to her manga counterpart, especially in the later events, who tries harder than any of the other witches..
Ending - Because there is so much being cut and the show being condense, one of the biggest fight in the manga has been turned into a very anti-climatic fight scene. The anime fails to show the strength of any of the character's resolve that is reveal during the fight. Instead, the ending wraps up the fight with most of the character doing things because they must without much reason beyond that. Also, the anime falls short on giving a good motive as to why Chisato protected Valkyrie which makes it hard to sympathize with Chisato and Valkyrie on top of completely, for the lack of a better term, crapping on Kotori's death.
All of this leads to....
Non spoiler summarization of the ending - Compressing 60 chapters into 2-3 episodes made the ending confusing and made many of the events seem like the plot is riddle with Deus ex-Machinas. The plot holes seemingly implied that most issues are caused by and resolved by "because magic".
Bad Setup for Season 2 - With the amount of changes and cuts, it's hard to imagine a Season 2. If Season 2 is to happen, it certainly won't be following the manga thanks to many of the cuts. Season 1 has set up Season 2 to go a completely different route on a weak foundation.
If you haven't read the manga, the anime definitely starts strong but as the show progresses it becomes confusing and devolves into a more generic shonen harem anime.
If you did read the manga, I suggest avoiding the anime or don't watch beyond 5 episodes.
Overall, the anime started strong and filled with mystery, and interesting characters. But the latter half of the show took a nosedive in quality due to the anime receiving a lot of cuts to fit 100 chapters into 13 episode. Had the anime stretch to 26 episodes, it would have been far better and less confusing.