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.
Complete Monster: A few of the Undead qualify. Isaka, the Catagory Jack of Diamonds Peacock Undead kidnapped hordes of people, got Tachibana hooked on a potentially deadly plant, killed his girlfriend for just trying to help him, and brainwashed a large number of people in his quest to create the "Ultimate Rider". He never shows a hint of remorse about this in any fashion.
The Catagory King of Spades Undead. He runs around, causing pain and suffering because he likes to. He ultimately steals of of Hajime's cards to reawaken the Joker, an Omnicidal Maniac who could destroy all life on Earth. Why did he do it? For fun.
Ironically, the worst monster in the series is a human! Hiroshi Tennoji arranged for the Undead to be unleashed, knowing full well that a massive amount of death and destruction would ensue. He brainwashed Trial B, who was meant to watch over his creater's daughter, for his own selfish purposes. He ultimately created the Kerberos Undead, watching gleefully as it slaughters the scientists who helped create it. He then fuses with it in an attempt to win the battle fight, trying to kill the Riders himself. Why did he do all this? Because he wants to win the Battle Fight himself and become the God of the new world he will create. He does think he can make the world better, though - his version of humanity will be more peaceful. The problem? All current humans would be erased in favor of his new humanity.
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.
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.