D.Grayman is many things but it isn't a horror manga. It tries very hard to be horror at the beginning but as the story progresses it loses this audience and then falls into the pratfalls of becoming a generic battle manga with meandering plot lines, heavy handed flashbacks, and blatant fanservice.
Allen is the rare type of protagonist that gets less interesting as the plot progresses. At the beginning he shows intelligence, confidence, and initiative. These traits quickly disappear when the author decides it would be better for Allen to be a pinball, awkwardly bouncing back and forth between the Noah and the Order whenever the plot demands. He also has one of the stupidest power ups in shonen which consists of a flamboyant clown uniform with a stupidly large sword he can't wield because his left arm is gone. Funniest of all this story actually turns ukefication into a plot point as Allen's progressively bishonen appearance not makes only makes him more prone to losing but means he is turning into a girly manchild named Neah. If he isn't fujioshi pandering i don't know what is.
Speaking of girls, the author continues the proud tradition of female shonen authors with awful gender ratios. There are only three female characters worth mentioning and each is badly characterized . First is Lenalee who characterization screams audience surrogate. One of the best fighters, has every male attracted to her, and secret yaoi fangirl, I'm really surprised why she isn't the main character. Her lack of flaws and tendency to overshadow less developed female characters made me really hate her. The second, Miranda is a kind but sorely undeveloped, ignored, and completely irrelevant arc only character who was later reintroduced for some unforeseen reason. The third, Road, speaks in almost babyish dialogue which is not only goofy but sadly matches the group she works with, the Noah.
I don't know what the author was thinking when she replaced terrifying Akuma with the Noah, a bunch of tattooed bishonen who are only slightly better at hiding their gay pedophilia than Black Butler. When they aren't awkwardly flirting with Allen they are speaking cryptic exposition or having pointless cutaways that slow the plot to a snail's pace. Their goofy personalities and Kuduzu style plans kill any tension and make the story feel like a Hetailia battle manga than horror.