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.
Contractual Purity: Averted; while pictures of Takayuki Tsubaki in a gay porn film were made public, he was still kept as the lead, and even returning in a couple episodes of Kamen Rider Decade.
Ho Yay: A lot between Kenzaki and Hajime. Especially given the ending. And even 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).
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.
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.
Women in Refrigerators - The fact that female Riders end up dead - 100% unbroken at the time of blade - is toyed with with Larc in The Movie. She dies, yeah, but all three Movie Riders are eventually killed off. As such, it doesn't end up seeming like it happened because she was a 'she.' She fares better in Decade, though.
Although that was to be expected, what with what happened in the previous movies.
Then there's the time the four series Riders reunited near the end of 2011. While it turned out to be for promoting a Bandai Premium line of show-accurate Rouse Cards, it still didn't stop the fans from guessing about them making an appearance in the Kamen Rider/Super Sentai crossover Super Hero Taisen.