First off this isn't Elfen Lied. Yes it's by the same author and has various similarities but it's not meant to be a duplicate and doesn't necessarily appeal to the same crowd. This isn't even really a horror/gore series, Inuyasha had more gore and it isn't considered a gore series. Brynhildr is primarily a dark drama with light comedy(despite not being tagged comedy).
Second this series was admittedly rushed and if you read the manga you would have been disappointed that they skipped a significant amount of storyline and character development. Would this series have been better if it was 24 episodes and stuck to the manga? Absolutely, I'd probably be giving it a 10 if that were the case.
From the writers perspective it's a toss-up if you were sure you weren't going to get a second season would it be better to make a developed series that stops halfway or a rushed series that's complete. Hard to say, but what I can say is that I've seen complete series that weren't nearly as well developed as this and they weren't getting ratings of 2.
Now onto the storyline itself. The basic plot is fairly generic, MC protecting a group of girls from an evil organization of course it has it's own spin on this. Nothing really great or bad there. I do think the blending of comedy and drama was one of the strengths in this series, some may think it was a waste of time in an already rushed series, but that's part of character development. It's the light-hearted comedy moments that endear you to the characters so you actually give a crap when they're about to get killed later. I've seen people complain on other series that all the friends do is rescue each other when they're in danger and never actually have fun so why are they friends to begin with? That's why this series tries to show that they have happy times when they're not fighting to survive. They're lives aren't constant fighting and carnage. If you're looking for nonstop action and gore than your better of watching Deadman Wonderland.
I will say the series had a bit more fanservice than was necessary though I find that to be the case in a lot of series. I thought Murakami was a good lead better than 80% of the MCs I see. He wasn't Lelouch but he was one of the intelligent strategist types and was fairly courageous and competent. Neko was pretty weak and generic overall unfortunately and didn't get much development. I did like Kana and Kazumi in particular both were fairly interesting and had some development. Kazumi in particular lost a lot of development from the manga but still ended up getting more than Neko. Hatsuna on the other hand was introduced at the end and got almost no development. Kotori was pretty annoying, even in the manga, I tend to think the writers didn't bother with her development (for reasons that would be apparent if you watched the series).
The villains were pretty interesting a stoic mad-scientist trying to play god and a psychotic yandere. Not unprecedented but still intimidating enough to take seriously and a still a better motive than the standard world domination.
I though the art was pretty good again nothing new and amazing but still better than a lot of series. The opening song was a favorite of mine and I thought it fit the series perfectly though the second seemed pretty random and didn't work as well.
Overall if you ignore the manga this is still a good series worth watching, it won't necessarily make your top 10 but its a solid series and still better than most of the stuff out there.
If you have seen the series I would highly recommend reading the manga because it does have much better pacing and development and very well may make your top 10
