First of all, the premise is certainly unique. A super powerful creature threatens to destroy the world, yet he wants to teach a middle school class? It's a concept that will leave you wondering what's going on, and this is something the series uses to keep you going: What are Koro-sensei's secrets, and when will we discover them? And I can say that while you do have to wait quite a while to find out everything, it's certainly worth it. There's a drip feed of information that we get throughout the story, and I do think this was the correct way to do it, rather than dump a ton of exposition on us all at once.
Aside from Koro-sensei's backstory, the series mostly focuses on Koro-sensei helping his students overcome various challenges they face in their lives, which helps to develop both the students and Koro-sensei. We get to see Koro-sensei acting like a real teacher, helping the students deal with real problems. Things do change up occasionally to something a bit more serious, such as an elaborate assassination attempt that the students have planned out, or the students having to deal with something the school principal has planned to sabotage them.
Now the story is definitely not perfect. In the beginning, I found myself not enjoying this series as much as I thought I would from the hype it gets. The main problem I feel the series had was consistency -- it would fluctuate between exciting and mundane. In the end, it created the feeling that every time it would start to get really good, it would immediately lose all of its momentum as it drifted back to something much less interesting. This problem is gradually reduced as it goes on, but it was a huge issue that kept me from really getting into the series for a while.
And what is the story without characters? And oh boy, does this series have a lot of them. It's extremely challenging for a series to develop a large supporting cast without making pacing drag or getting formulaic (i.e., a pattern of each character getting a story all in a row). The story calls for an entire classroom of children, their teachers, and more. Unfortunately it just wasn't possible to flesh all of these characters out in any sort of reasonable amount of time while also moving the story forward. Instead, most of the attention is placed on just a few characters: Nagisa, Kaede, Karma, Koro-sensei, Karasuma-sensei, Bitch-sensei, and the school principal. While most of the other characters are at least touched upon, they are mostly relegated to the background. What was nice was that even though most of their development was unseen by us, they would continue to appear and have consistent traits, so clearly the author had personalities for all of them. That all being said, the characters who do receive the most attention are all very interesting and well-developed. They each have different motivations in the situations we find them in, and it's never really unclear why a character is acting a certain way (barring when that mystery is a plot point).
I found the art to be pretty standard. Certainly not bad, but nothing that made me sit back and truly admire a page or panel. Unfortunately the large supporting cast combined with the limited colors available to a mangaka can sometimes make it difficult to figure out who's who, at least for the background characters. The main characters are all visually distinct and you'll never find yourself mistaking them for anyone else, but I often found myself saying, "Wait, who's that again?" when it came to some of the lesser characters.
Overall I think Assassination Classroom is a series that, while flawed, is still an enjoyable ride. The story really gets going toward the end when a lot of things are finally revealed to the reader. It's just that getting there can be a bit tough at first.
