An interesting enough premise, but Blue Exorcist turns out to be just another school-setting shounen with a main character with the typical daddy complex and a shallow/unfinished romance.
The colors were bright, which is always enjoyable, but the character designs were lacking and some features came out awkward-looking. Ex: Shura's oddly boxy midsection and Yukio's unusually wide face
None of the OSTs stood out to me, but I loathed the jazzy opening.
I really enjoyed Rin and Shirou's relationship, and Shiemi's smidgen of character development was nice to see. But there was little depth in the characters otherwise. Instead, the writers took the easy route and focused on Izumo's (spoiler) easily-predictable somewhat transformation and Rin and Yukio's unorthodox relationship (which came off as awkward and forced to me). Though I must applaud the series for (spoiler) letting Noriko go, which shows some resolve.
Concept & Execution: 5
The daddy complex was nothing new, though I thought the religious overtones and occasional references were interesting and almost Neon Genesis Evangelion-esque.
I was rather let down by the series after hearing so many good things from friends. The first few episodes are still wonderful and emotion-filled, in my opinion, but the rest of the show fails to follow-up and the conclusion is horrifically anticlimactic, (spoiler) with the exception of the relationship between Rin's mother and Satan, which I found surprisingly touching.
Overall, it feels like a textbook shounen that could have taken an edgy turn with the religious overtones and instead wimped out.