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.
YMMV: Nabari No Ou
Awesome Music: "Crawl" and "Aru ga Mama" (the anime's opening and second ending song, respectively). "Yureru Kokoro" from the OST also qualifies; it's simultaneously strong and mournful and enhances the emotional impact of several scenes.
Complete Monster: The Mysterious Cloaked Guy(revealed to be Inasa). He kills 4-year-old Miharu - in a pretty gruesome way, no less. Later, he reveals that he's not only another immortal like Aizawa and Shijima, he's also actually Fuuma Kotarou...who's been tricking, playing with and manipulating everyone from the start because he won't let anything stop him from possessing his "lovely Shinra-san", though as he said, and Aizawa admits, he has been telling everyone all along that he wanted the Shinrabanshou...
Fashion-Victim Villain: Raikou for sure, except for the outright "villain" part. His work clothes include something like a denim vest, a hakama over jeans, and sandals. His casual clothes are canonically recognized as really, really, really terrible. He has seriously considered buying a coat of gold sequins. And one with giant stars across it. And has worn a shirt with huge feathers hanging off the back.
Foe Yay: Miharu and Yoite start out like this, but are really, really bad at acting like foes, and it quickly blossoms into full-fledged Ho Yay, especially after Miharu's Hazy Feel Turn. There's some minor Foe Yay between Kumohira and Yukimi too. Not to mention the rather creepy scene where Hattori is practically hitting on Yoite - "You really are diligent and smart... as well as beautiful."
Het is Ew: Some fans were freaked at the possibility that Yoite could be a girl. He's actually a hermaphrodite.
Subverted with Kouichi/Raimei being one of the general fan preferred pairings.
Raikou nearly crosses it when he tries to kill Raimei, only to be stopped by Gau.
Narm: In the last episode of the anime, a smiling image of the dead Raikou appears in the sky as Raimei and Gau are talking about him. Given how old and frequently parodied that device is, it's hard to take it seriously.
Nightmare Fuel: Among other things, 4-year-old Miharu getting his face blown off and Asahi's body falling to pieces.