It is generally accepted that anime based on manga often divert from the original storyline, sometimes by very little, as in Fullmetal Alchemist: Brotherhood, and in other cases, by a huge margin, as in Fullmetal Alchemist. Other stories make it worse by stuffing the show with ridiculous filler episodes.
Yu Yu Hakusho falls somewhere in between, with the anime staying mostly faithful to its source, but where it doesn't, the discrepancies are vast. The list given here communicates the ten most glaring instances of alteration observed between each version of the narrative.
10 Koenma Doesn't Have A Sidekick
One of the few anime-only roles in the series is that of Saotome Jorge, a goblin-ogre fusion with lilac skin and a cheeky grin. He plays Koenma's personal assistant, which really means that he has to bear the brunt of his boss' anger (even if the fault lies with the latter.)
Jorge's hilarious antics add the informal twist required to balance between the show's darker aspects, and he plays the part with astounding skill. In fact, audiences learn in the penultimate episode that Jorge has been the narrator all along, suggesting that he might be a metaphor for the mangaka himself.
9 Yusuke's Various Vices
Although Yusuke is the definitive delinquent: cutting class, avoiding homework, morosely squatting on the school terrace to pass the time, he has a gentler side to him as well.
The real story begins when he gets killed in a car accident while saving a child, which is the main reason that he isn't immediately shunted off to Hell as anyone else would with his track record. The manga version is a lot worse before his death; he has a penchant for nicotine, alcohol and gambling, which have been entirely excised from the TV show to make it more family-friendly.
8 Botan's Job
Botan might be a Spirit World guide, but she quickly strikes up a friendship with Yusuke and everyone else, becoming a core part of their gang. In the manga, she adheres to her task a lot more strictly, and doesn't come to Yusuke's assistance when he has to battle Gōki, the Soul-Sucking Demon responsible for consuming the spiritual essences of little children around town.
Further, Botan watches her pals duke it out during the Makai battles (although camouflage) in the anime, which also never happens in the original source.
7 Yusuke's Mother
Yusuke's Mazoku blood must have been obtained through his mother, Atsuko, given that her general attitude is identical to the woman that Raizen, his ancestor, once fell in love with.
However, the manga character has the ability to perceive the spirit world as well as any lurking demonic entities, and she is also in the audience watching her son face off with Younger Toguro during the Dark Tournament. Interestingly, Atsuko is linked to the Japanese underworld, because her actions feel like those of a Yakuza leader (according to Yusuke.)
6 Kuwabara & Yukina
Yukina, Hiei's baby sister (although they barely look the same species), plans to stay with Kuwabara's family at the end of the manga storyline, just as he begins to find himself the center of attention for the first time as a preppie high schooler at a prestigious technological academy.
Yusuke playfully teases him about the consequences of having his crush around him all the time, specifically in terms of his academic accomplishments. The anime Yukina stays with Genkai instead, aiding her in the management of her temple and the vast estate surrounding it.
5 Kurama's Moves
Strangely, only one of Kurama's techniques is designated anime-only — the Okunen Ju Roots that he employs in one of his final matches of the series.
The manga, though, contains several specialized abilities that were never adapted for the screen, for instance, Giji Kuricha, a horrifying botanical punishment involving a parasite; Ha Ken, a blade that he generates from a leaf; not to mention that Kurama can also travel to the Spirit World at will using Astral Projection.
4 Sensui's Motivations
Sensui is the unpredictable villain of the third arc, not for his invincibility as much as his mental instability. The anime plot has him feel remorseful for all the demons that he had been "forced" to murder during his stint as Spirit Detective, and therefore desires to travel to the world of his victims, so he can sacrifice his life there: a twisted form of self-flagellation.
The manga Sensui also insists on making the same journey, but he doesn't have any specific reasons for doing so. Well, except for the faint hope that if he passes away in the Demon World, then perhaps he might one day reincarnate as one of them.
3 The Makai Tournament Battles
Yusuke decides that the next monarch of the Makai will be "elected" through a series of matches, dubbed as the Demon World Tournament. Thousands of combatants participate in the fights, but only the important characters (obviously) end up making it to the final roster.
Some of these battles are only found in the anime, such as Hokushin vs. Shishiwakamaru, Kujo vs. Toya and Kurama vs. Shigure, while a few were exclusive to the manga: Toboso vs. Zakuro and Yomi vs. Koukou.
2 Koenma's Rebellion
Unlike the anime, the manga takes a sharply political turn when little Koenma ousts his own dad, King Enma, for horribly unethical behavior. For example, Enma fakes documentation regarding human murders, making it look like the real aggressors were demons when in fact most of the culprits were ordinary people.
Further, he was said to abduct weaker demons and manipulate them into wreaking unnecessary havoc in order to improve his own prestige. Thankfully, the Spirit World is currently under considerably better guidance.
1 Genkai's Last Days
Right before the conclusion of the story, Genkai invites her friends over for tea and offers them her property in the form of inheritance, should she die at some point. Everyone else is stunned by this revelation, but they return to their normal cheerful selves soon after.
Genkai's manga role ends in death (of old age, most likely), but the specific terms and conditions of her bequeathal remain unchanged. More importantly, it seems that she still wields some power even in death, as she takes control of Puu's body in order to deliver a message to Yusuke.