All-Star Cast: The anime has a pretty impressive cast, for its genre and category. In the voice commentary the director said he'd been surprised that some of the more high-profile seiyuu (such as Rie Kugimiya, who cameos as Azazel's girlfriend Kiyoko in the first episode, or Keiji Fujiwara) accepted his offer. We also learn that at least one famous seiyuu refused to even audition when he learned what kind of anime it was.
Executive Meddling: With the manga being what it is, the anime has been on the receiving end a lot during production to be fit to air on TV. In interviews various staff members asked Kubo Yasuhisa to write stories that can be adapted easier for TV.