The Chris Carter Effect: Affected a lot of fans. What was Sagadalius talking about in the Devil's Trill arc? What does Konoe have to do with it? What happened to Ruka? What was up with the hints of incest in volume 2 of the manga? If not a human, what the hell is Tsuzuki? Is the child in his head a real entity or was he just imagining it? Why doesn't he remember anything about his stay in the hospital? How did he manage to actually die after failing so many attempt because his body was just indestructible? What was the significance of the panel where they show some of the Count's face in the manga? What's the deal with Tatsumi's mom? What's the story behind Tatsumi and Tsuzuki's first meeting? Is Enma evil or not? Complete anime series and we're at volume 11 of the manga, and none of these questions have been answered yet, and they keep building as we continue through the latest arc. After nearly a ten year hiatus, some fans have abandoned ship.
Complete Monster: Dr. Kazutaka Muraki, while handsome and charming, has, as of the start of the series, already spent several years systematically kidnapping, dismembering, and vivisecting small children, raping adolescents and infecting them with slow, agonizing fatal diseases, and seemingly murdering every grown woman he comes across, whether to use them in scientific experiments to attract the attention of the Shinigami or just for fun. Muraki also specialized in Mind Rape, whether it's mentally torturing a girl madly in love with him into developing split personalities, mind-controlling suicidally depressed pop stars into becoming blood-drinking monsters, or stalking and harassing one of the Shinigami with so much misdirected guilt that he finally manages to drive him into a catatonic state, in which state he fully intended to vivisect the guy and use his powers to bring Murakiís older (half-)brother Saki back to life, just so that he could kill him more slowly and painfully.
Let's see, there's Hisoka, Muraki (the most blatant one), Hijiri, the Count (just as blatant as Muraki), Tatsumi, Watari (a one time Stupid Sexy Flanders case), even his Shikigami... Yup, everyone's gay for Tsuzuki.
And then there is a particular Shinigami, Terazuma, that is 'allergic to girls' turning him into a "Giant Afghan Hellhound" if touched by them. Hell, just looking at 'girly' Hisoka set Terazuma off!
Not just looking, attempting to kiss. It was a contest.
And he never stood a chance, especially while trying to imagine him as a flat-chested girl.
Les Yay: Self-identified lesbian Rika seemed to have a thing for Chizuru, while Saya and Yuma go around groping each-other in onsens.
Jerkass Woobie: Nagare. It's true he was negligent of, if not downright abusive towards Hisoka in ways, but it can't be denied that the dude's life has sucked.
Launcher of a Thousand Ships: See Ho Yay above — everyone, but especially Tsuzuki. There's enough backstory, mental trauma, and suggestive panels in this series to give canon backing to pretty much any ship. Take your pick. There's Hisoka, Muraki, Tatsumi, The Count, Hijiri, Byakko, Touda, Suzaku...
Marty Stu: One could make a case of this for Tsuzuki; he has a Dark and Troubled Past that's played up as being even darker and more troubled than any of the other characters, has an unusual eye color, is extremely powerful, is nice to everyone, has almost otherworldly handsomeness and is loved by everyone (even the villain!). That being said, his derpiness seems to balance this out a little.
Muraki crosses it so many times that it's hard to keep track. But raping and murdering Hisoka is a good start...
Maria Wong's stepmother crosses this when she has Muraki bring Maria back from the dead despite the fact that Maria committed suicide to escape from her stepmother's abuse, and did so just so that she could abuse her more.
If killing the first Hisoka as a newborn didn't put Iwao here, desecerating her grave and stealing her remains certainly did.
Ron the Death Eater: Yes, Nagare Kurosaki never was a good father to Hisoka. He is still not the horrible and heartless bastard that overprotective Hisoka fans think he is.
Wangst: Tsuzuki wangsts a bit much over every single person's death considering that he's an ancient god of death. This is actually explained in canon: he feels so much pain over each person's death because he doesn't think that someone who committed suicide has the right to end the life of someone else who wants so much to live.