Character Development (Especially Eren)
It is easy to forget at the outset of Attack on Titan that the majority of the characters in the spotlight are barely even teenagers. As such, they are prone to being whiny, scared, and incredibly stupid.
Just about every episode, I found myself shaking my head at the decisions Armin, Eren, or the droves of titan-fodder supporting characters made that put them in avoidable situations.
Over time, though, each character grows a distinct personality, offering a particular skill set that is vital to the war against the Titans. Even the antagonists get ample screentime, making the conflict between each side that much more compelling.
I’m especially impressed with Eren’s growth, as he evolves from a cliche, overpowered, annoying anime protagonist to a complex and flawed main character.