Ao no Exorcist is one of those shows you will either hate or love, without much of a middle ground. The reason for this division of opinion is the amount of shounen cliche that makes up roughly 80% of the story.
Story: Like I said, Ao no Exorcist is pretty much one shounen cliche after another. Everyone in the cram school has some special ability or they are pretty much there for comedic relief (or random plotpoint when necessary). The main character is an orphan with ties to pretty much every secret society in the anime for one reason or another, but he has no idea of that until he is abruptly made to realize that "Oh my god, I have superhuman abilities". The main action in the anime is centered about Rin learning to use said superhuman abilities without turning into a demon.
Animation: The animation randomly switches from the light-bathed halls of the exorcist school to the dark, and rather eerie battles with demons. Movements are fluid and the settings are intricate. A lot of detail is given to each character, regardless of their importance, so the animation is top notch.
Sound: The OP's set a pretty upbeat and quick tempo for the anime, which it needed considering how much fighting takes place. The soundtrack enhanced the fighting scenes enough to engage the viewer.
Characters: The characters were a bunch of teenagers with enough personality quirks to set them apart from each other. Still, they tended to fall into certain cookie-cutter character molds. While I liked the dynamics between the characters, I wanted them to have a little more depth.