Story 9/10
The story is pretty linear. The two terrorists place bombs, and announce them with anonymous videos, where they also gives riddles as to where these are located so that they can be defused. The main plot is therefore driven by the terrorists actions setting these bombs, and how the police (in particular an agent, who is the main counterpart to the terrorist duo) solve the riddles.
However, along the story an additional group is introduced, which complicates the intellectual conflict between the police and the terrorists, and escalate the action until the finale. Along the way we also get to learn about the past, and thus the motives, behind the terrorists' action.
Although there are no major plot twists and unexpected moves, the story does a good job to keep you hooked. The intellectual and psychological conflict between the terrorists and the investigator feels sometimes a bit rushed - it is nothing as brilliant as Kira vs L in Death Note for example - but overall it is an interesting plot.
Having said this, the main plot-line is really not the only good (or even main) thing why I liked this series. Several themes are approached on the side of the main plot, whether that's political themes (the complex tension between Japanese nationalism and their subordination to the US), power relationships, human emotions, love and friendship, which make the series much more interesting than a simple plot-driven sequence.
Art 10/10
I have to confess I am no expert, but I really found the drawings of the characters really good. Although their traits are pretty conventional (the "clean-face" kids, the psycho-antagonist, the scruffy police agent), I felt they were perfect to represent all the characters.
But especially good was in my opinion the design of the scenic background. The settings are really realistic, not too dark, not too "cartoonish". Some shots in specific scenes were great.
Animation, as far as I can tell, was flawless.
Sound 10/10
Voices were good just nothing spectacular. Sound effects also good (you'll hear some cicadas like in Steins Gate:) ).
But the soundtrack was a masterpiece. Perfect to characterise the scenes.
Character 9/10
Although the series is not really about character development, as the characters are indeed pretty static, the attachment I developed to the main characters I think was the main reason why I loved this series.
Although the premises which lead the terrorists' actions are relatively straightforward, their characterisation is in my opinion pretty original. There's plenty of anime where dark childhood lead characters to be either psychopaths (e.g. Johann in Monster), genius weirdos (e.g. L in Death note), or seemingly cold but good-hearted (e.g. Emiya Kiritsugu in Fate Zero). In contrast in ZnK the characters are simply "good guys".
Now this may seem a dull thing, but in my opinion it was the most refreshing part of the series. The two terrorists have been wronged by life, and simply want to show something to the world. But the world itself is pretty screwed up, which in fact is what brings together the main characters and their counterpart.
The reason why I don't give this a 10/10 is because the female characters were actually much less effective. They are also suffering from a tough life, but unlike the main characters and their counterpart in the police, they react in a much more "conventional" (for anime standards) way. You have the girl who used to be bullied, and creates tons of problems due to her ineptitude - her interaction with one of the main characters is however more interesting -, and the crazy psycho who is also being used by the evil characters in the world, but has her own motives - which we actually never really understand.
Enjoyment 10/10
I really enjoyed watching it. The anime is about right in its pace, not too slow not too fast. But the main reason I enjoyed it is not so much because for particularly intense action scenes, but rather because of the emotional attachment that I developed to the characters. You want to know what is going to happen to them, and you want to know how.
Overall 10/10
ZnT is really a good anime. It makes you think - through the many themes that are touched upon -, it makes you happy, and it makes you cry too. I really recommend it.
