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.
And before that, we had Destiny's Play. As well as it's Re-Union Edit.
Saga has his own awesome Leitmotif: Roots of the King.
Generally, everything TETRA-FANG (including Koji Seto/Wataru himself) made. To give one an idea of how awesome the music of this series is in the Heisei period: apart from Den-O's all-too visible Wolverine Publicity, only the Kiva soundtrack has proved popular enough to warrant a reunion concert years after the show.
Badass Decay: When Kengo is given IXA, Nago loses a lot of his cool. And gets punched a lot.
But then he gets it back, and he spends most of the rest of the series beating the crap out of the second most powerful Fangire, Bishop- a Fangire who handed Kengo his ass on a platter.
Rook, who spends all of his free time making up "games" for himself that generally go along the lines of, "Let's see if I can kill X amount of people wearing Y in Z amount of time." He is also responsible for the near-genocide of, at the very least, the Wolfen, as well as the death of Yuri Aso's mother, Akane. His "death" at Yuri's hands (and his total annihilation at the hands of Megumi) was quite karmic indeed. Unfortunately, he ultimately ends up doing a Heel-Face Turn. Why? Because he has nothing better to do and wants to go to Heaven. He succeeds in doing so.
The 1986 King was monstrous himself, albeit on a more personal level—forcing Jiro (the last of the aforementioned Wolfen), Ramon (the last of the Mermen), and Riki (the last of the Franken) to become Empathic Weapons and developing a full-blown hate-on for Otoya, up to the point of threatening his own son's life if his wife sees Otoya again...the same woman whom he told was not worth killing because he had no feelings for her. Kivat II, who is meant to be the King's loyal Transformation Trinket, is so horrified by King's threat to kill his own son that he pulls a Heel-Face Turn to let Otoya become Dark Kiva to defeat him.
The Rhino Fangire was rather nasty. He tricks young people into believing he'll help make their dreams come true to lure them in so he can steal their life force. After killing them, he takes an item from them that relates to their dream as a trophey to put in his 'Dream Graveyard". His choice of prey is based solely on a love for crushing the dreams of young people and nothing else. Even Wataru, who had generally tried to find the good in everyone, even Fangires, found his actions unforgivable.
Epileptic Trees: There were plenty, of course, but one of the most common got subverted surprisingly thoroughly. Since 1975's Kamen Rider Stronger, it's been standard operating procedure for seasons of Kamen Rider to have females who get powers to die. Since it's made clear very early on in Kiva that Yuri was going to die soon, most fans assumed that this would happen as a direct result of her getting and using the IXA powers. Come mid-season, she gets the powers, uses them in battle, kicks complete and utter enemy ass... and nothing happens. The show never really explains how she died; many fans at this point assume something relatively mild, like a car accident.
Fetish Fuel: Jiro and his silk and leather outfits, Ramon for anyone with a thing for shotas (a fandom term for his actor is "Shotabait"), Maya's ENTIRE OUTFIT, and King's outfit. A lot of the things Taiga and Wataru do in episodes are particularly suggestive (there's some serious Ho Yay and Bro Yay going on here).
Growing the Beard: The story really picked up speed after about 20 episodes, with the introduction of Mio and Maya.
Hilarious in Hindsight: ep 27 opens with Kivat saying something about time travel being a possiblility, even stating 'I'd like to return to that day'. Tomokazu Sugita would later return for the second season of Haruhi Suzumiya, infamously known for Endless Eight...
Speaking of Kivat's voice actor, one episode starts with him attempting an impression of Norio Wakamoto, referencing his role in The Movie. Sugita's dubbing of Ohja/Strike is basically the same thing. And then there's this.
Saga's finisher involves impaling and hanging fangires before ultimately causing the victim to explode in tremendous pain. One could actually feel sorry for the Horsefly Fangire for having to die in such a brutal way.
For starters, the show's basic setup (namely, the 13 Demon Races) should have allowed for a lot of interesting characters and significantly impacted what the world is like. Instead, we only really got to deal with the Fangire and The Masquerade is strong enough to ensure that next to nobody knows about the Demon Races, even when the city gets attacked by Sabbats.
It even goes down to basic features for the Riders. IXA only uses the fake Fuestles once in the entire show, likewise with Kiva using some of his powers (Dogabaki and Shoodoran for starters) just once or twice.
Wataru's status as a half human, half Fangire. He doesn't suffer any negative effects because of his Fangire heritage until Taiga and Bishop stir it up once in an attempt to get him to come to their side. He's also not very strong (at one point it is shown that he can't do more than ten push-ups) and does not possess any sort of unique ability born from his monstrous heritage (when not transformed, that is). To be fair Wataru doesn't learn about his unique status until late in the series, but the writers could have made Wataru more interesting if they gave him some sort of weakness or problem that resulted from this and made him struggle with it until he learned about his parents and learned to accept who he is. Wataru does accept his heritage, but the only thing he has to show for it is Kiva, his status as the new King and a lot of painful memories. As it stands the only difference between a human and a Fangire/Human mix is that the mixture can use things that would normally only be accessible to a Fangire (like Castle Dran) and would not have to worry about the negative effects of transforming with Kiva/Dark Kiva.
The existence of people with mixed heritage arose much earlier in the show - Jiro wanted to marry Yuri so that she would bear his children and help repopulate his race. This plotline was dropped after Jiro's true self was uncovered and Yuri revealed that she was in love with Otoya.
The fact that nobody mentions what exactly happens to Yuri to cause her death somewhere in between 1986 and 2008.
What an Idiot: Yes, Bishop, do tell Taiga that you killed Mio instead of shutting up and letting Taiga continue to assume that Kiva did it, therefore getting rid of him without having to lift a finger. No wonder you are anEvil Genius!
What the Hell, Hero?: Taiga kills Maya, the mother of both him and Wataru. Wataru doesn't seem to be upset by this fact and instead eagerly accepts his help in the fight against the revived 1986 King. It is shown later that he didn't really kill her, but still, that was way too accepting.