These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The anime's opening "CORE PRIDE" by old favorite J-Rock band UVERworld.
The second opening, "IN MY WORLD" by ROOKiEZ is PUNK'D is just as awesome as the first opening.
Not only the openings are awesome, the soundtrack itself deserves a mention, too. See for yourself.
UVERworld makes a triumphant return to the series with the theme song to The Movie, "REVERSI".
Cliché Storm: Be ready for for A LOT of recycled Shōnen tropes. Not that this is a bad thing. To the series' credit, many of the tropes are occasionally twisted in new ways to keep things fresh. It arguably succeeds in this end as well.
Meisa's Kuroki's "WiredLife" is another contender.
Ensemble Darkhorse: Renzou if the merchandise has anything to say about it. This guy is snuck into pictures alongside more central characters of the show when his fellow companions who've arguably been given more focus, Bon and Konekomaru, are nowhere in sight! There's even an official letter set where he appears more often than the main character, for some reason. In the official poll he ended up being the fourth most popular character in the series after Rin, Amaimon, and Yukio.
Fans seem to have taken quite a large interest in Amaimon, to the point he managed to place second in the popularity poll, while Yukio got first place and Rin ended up in third.
Juuzou and Kinzou, Renzou's older brothers, are very popular for being introduced not too long ago and with only moderate, and moderately important, screen time. In general, the Shima brothers seem to be very popular amongst fans.
Expy: Rin does have a resemblance to a certain Public Domain Character. Flaming Sword? Check. Blue fire? Checknote Obscure, but true, as the old verbal charm used to invoke him refers to his "flaming sword of cobalt blue.". Incredibly protective? Check. Willing to help all in need? Check. A tendency for arrogance? Check. Popular? Fandom says yes. Who does he greatly resemble? No one else but Archangel Michael himself! And the recent Story Arc introduces the Demon King of Light...aka Lucifer as the current Big Bad.
Hate at First Sight: A meta-version; within a few panels of Angel's introduction, the fandom already have him on their list of most hated characters in Blue Exorcist. And he's above Satan. Yes, he's that much of a jerk. Seems to be slowly transitioning over to Love to Hate, mostly because he's an idiot (worse than Rin as a matter of fact) and he ended up placing fifth in the official popularity poll.
One of the first filler episodes in the anime involves a Kraken attacking a coastal town. Guess what the first chapter after the whole Impure King arc involves?
It's funny think that Hiroshi Kamiya (Mephisto) spouts any German used in the show when Amaimon's voice actor, Tetsuya Kakihara, was born and raised in Germany and is as fluent in German as he is in Japanese.
The Gecko Ending of the anime had Satan attempting to join Gehenna and Assiah together through a large Gehenna Gate to create a "new world". Guess just what's been revealed to be the Chief of the Illuminati's plan?
Ho Yay: Bon/Rin, what with Rin going on about how "cool" Bon is, getting all wide eyed and blushy when he does something awesome or nice and then him getting the most depressed over Bon being angry with him over his Son of Satan status. And Bon doesn't skimp on it either; he blushes the first time he's nice to Rin, as a lot of Held Gaze moments with Rin, his treatment of Rin can be seen as rather Tsundere and his declaration of friendship comes across almost like a love confession. Which conveniently helps Rin get past a mental block...
Shiemi's strong desire to befriend Izumo, and willingness to be used as a doormat to that end. Izumo's tsundere and Defrosting Ice Queen towards...well, everyone, but especially Shiemi.
Izumo and Paku, on the other hand, seem to be nearly canon. Izumo initially had no interest at all in associating with anyone but her best friend Paku.
Saburouta Toudou. It doesn't help that he has straddled Yukio THREE times and that everything he says to him is a double entendre.
The Scrappy: Yukio was this at first for his lack of a personality and screen time. Subsequent development turned him into a Base Breaker and he won first place in the popularity with 2529 points, over 1000 points more than the runner up who got 1373.
What The Hell, Townspeople?: In order to save his friends from Amaimon, Rin reveals his secret nature. Despite proclaiming their friendship only 3 chapters (a couple hours In-Universe) before, his classmates start ignoring him in a silent Take That. In the anime this is short but in the manga Rin is ignored for several chapters (daysIn-Universe). Only Izumo still talks with him during this time and mocks the others for their hypocritical statements.
Although to be fair, he was revealed to be the son of Satan, the mortal enemy of all exorcists, and some of them had their own reasons for ignoring him.
On the other hand, it is still really harsh to do that to a friend that you've known for awhile.
Subverted in the anime with Ernst. According to him, he lost his face, his daughter, and his freedom all at once. It took YEARS for him to climb all the way back up, and the only thing that gave him the courage to do so was his photo of his grandchildren (whom he'd never met.) However, everything he said that he went through, aside from having his face burnt off, were all Blatant Lies.