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.
Isaka, aka the Peacock Undead, exemplifies how low manipulative bastards can go. As the first Category Jack Undead, he designed Rider System 3, a belt that allows the user to transform into Kamen Rider Leangle. Unfortunately, that same belt requires using an improperly sealed Ace card that contains the Spider Undead, meaning that its user would be mentally enslaved by the Undead (as later seen with Mutsuki). After designing the corrupted Rider System, he brainwashed many people that he was going to use as expendable slaves to create his "Ultimate Rider". When this plan failed, Isaka turns his attention to Sakuya Tachibana and attempts to retrieve him as a prototype user. Isaka knows about Tachibana's subconscious fear and takes advantage of it to curbstomp him. Isaka tricks his defeated opponent to getting addicted to Schuld Ketsner seaweed, lying that the aggression-inducing plant would eliminate fear. When Sayoko notices her lover's addiction, Isaka smugly kills her for attempting to clean up her lover. Upon learning about Sayoko's death, Tachibana loses his subconscious fear and eventually gives the bastard exactly what he deserves.
Creator's Pet: Mutsuki Kamijo. He's despised by the audience yet the staff likes him.
Fountain of Memes: In its native Japan; other than the Ondulish "language" this series Riders (especially Garren, for some reason) tend to cross over in other series fan art.
Ho Yay: A lot between Kenzaki and Hajime. Especially given the ending. Moreso given the tone and content of this song.
Memetic Mutation: A relatively unknown meme in the West (for obvious reasons), the Japanese have had a lot of fun with Kamen Rider Blade. Onduru-Go (roughly translated as "The Ondul Language" or "Ondulish"), is a meme originating from protagonists, Kenzaki's fairly heavy-handed enunciations. The meme name comes from the first episode where Kenzaki is supposed to be saying, Hontou ni uragittan desuka!? ("Have you really betrayed us?"), but ended up sounding like, Onduru ruragittan disukaa!? Since then, it's reached a fair amount of popularity as a Japan-based net meme and has in fact received reference within the Blade Alternate Universe episodes of Kamen Rider Decade, as well as a shout-out in Suzumiya Haruhi no Yuutsu. It got another nod in the net movies for Let's Go Kamen Rider (the 40th anniversary movie), where the phrase that inspired the meme is the title of the episode that focuses on Blade. Kenzaki, for his part, has since been labelled the Kenjaki, the Prince of Ondul Planet.
Another aspect of this meme are the Blade emoticons — ( 0w0), ( 0M0), ( 0H0), (<::V::>) — and chibi Blade's going "Wa~y!" (Mondegreen from the original opening theme's "BLADE BRAVE" line, and his primary Battle Cry).
Another famous one is the shot of Garren looking out silently behind a wall from episode 1. Whenever there is a Blade related fanart, be sure to look out for this.
Nightmare Fuel: The Leangle belt leaks spiders, which should creep anyone who has arachnophobia out.
Also, children could get easily freaked out by Mutsuki when the Spider Undead possesses him.
The Scrappy: Amane, due to being a Clingy Jealous Girl for Hajime, to the point that she absolutely cannot function without him around, making her attachment to him just creepy. There's also the fact that she's incredibly disrespectful to adults, including her own mother, and especially to her uncle.
Stop Helping Me!: Done entirely for humor in the Hyper Battle Video. A Trial has impersonated Kazuma, and he asks Tachibana and Mutsuki for help. Each of them in turn says "I know who the impostor is!", pulls out his personal weapon (while not transformed)...and attacks the real Kazuma, getting him shot in the ass and bashed over the head respectively.
Even worse is the novel continuation, where Kazuma has been Walking the Earth for 300 years and has become a Death Seeker because of all he's had to give up. Even worse, he can't die at all, since if he were to die Hajime would be unsealed.
Wangst: Mutsuki's childhood trauma from being kidnapped and locked into a safety box as an infant was brought up early in the series, apparently reinforcing his resolve to become a Rider...at least until the trauma's resolved and the Wangst is all that remained.