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.
The RX/Black arc has a BGM of Awesome which plays several times throughout (most notably in the double henshin and Double Kick), which is actually a piece of production library music called "Millions of Me" from VideoHelper and has been played on the gameshow Minute to Win It.
Big Lipped Alligator Moment: Double's appearance in All Riders vs. Dai-Shocker; he simply rides in, hands Shadow Moon his ass on a platter (after SM beat Decade AND Rising Ultimate Kuuga at the same time), sends him flying into the Dai-Shocker HQ, then rides off. However, the aftermath of the BLAM gives all Riders an extremely wide opening to perform the All Rider Kick.
It does later get a nod, however, in Movie Wars.
The Kabuto World story ends with Kivaara facing off with a Worm mimicking Kivaara, for no reason other than to have a Worm mimicking Kivaara. It gets weirder if you're familiar with the concept from the original series. note Typically, Shapeshifter Baggage is only loosely employed so that Worms, being human-sized, can only impersonate other humans - but Kivaara is the size of a baseball.
The first sign of Diend's powers shows up during a fight in Blade's world, where suddenly out of nowhere Todoroki appears. He plays his guitar solo, strikes his guitar into the ground attacking everyone, then disappears.
Cargo Ship: thanks to what happened to him in Diend world, Yusuke/Kivaala.
Epileptic Trees: Not only is the typical life expectancy of female Riders subverted, but two female Riders (Akira in Hibiki and L'Arc in Blade) are brought back just to have them retconned with a much better fate. Unfortunately though in the Rider War...
And then the Final Chapter movie has Natsumi becoming Kamen Rider Kivara and they even give the original quasi-female Rider sidekick Tackle a decent sized sub plot as way of homage.
Though, Natsumi prior to being Kivara was Den-o for one episode, and was killed off a couple of episodes later. She gets better.
Speaking of Sentai, the first fight in the Shinkenger crossover with both Rangers and Riders involved Shinken Gold and Diend. Care to guess which Ranger is among the evil duplicates created by Diend's (outright villainous) Kaizoku Sentai Gokaigerexpy Basco in his debut episode?
About Gokaiger again, though more subtly: the first Shinkenger episode of Decade was one of the very few times in Decade where the Kamen Rides (both Diend's and Decade's) had a thematic link to the episode's theme (Shinkenger, Ryuki and Den-O were all written by Yasuko Kobayashi); Gokaiger turned out to be far more liberal in its usage of such thematic transformations into past heroes in episodes with alumni guest stars.
At the beginning of the Den-O World arc, everyone assumes that Den-O's Final Form Ride is supposed to be DenLiner since that's how the card shows it...then later on it's revealed to be Momotaros himself. And then in Final Chapter, the DenLiner thing becomes reality.
The appearance of King Dark near the end of All-Riders is funny in retrospect, since it means two personifications of the Great Leader note the second one, obviously, was Decade are duking it out.
In the World of Hibiki, if you're an Oni and lose your sense of justice and seek only to be stronger, you turn into a Makamou. Change the words Oni and Makamou to Magical Girl and Witch respectively and you get what happens to Sayaka just a few years later.
Also replace the words justice and seeking to be stronger with sanity and refusing Grief Seeds respectively.
To make this one even better, the writer of the show went on to write Kamen Rider Gaim, in which Ryoji Hase goes insane and becomes an Invess.
Nightmare Fuel: Judging by her reaction, Momotaros-possessed Yuusuke was this for Natsumi.
Some of the deaths in this series, particularly Great Shocker's assault on The Multiverse, where people get killed onscreen.
Out-of-Character Moment: Kuuga is the original (Yusuke Godai), but he acts like a more airheaded version of Yusuke Onodera who can't even remember Decade's name (at one point he thinks it's Dickens). In the Den-O world, Momotaros is polite and refers to the other Taros by their actual names rather than his usual nicknames; Urataros is described as "the blue Ryutaros".
Shilling The Wesley: The original Kabuto (Souji Tendou) is all but directly called the strongest Kamen Rider ever, and he effortlessly beats down Decade and Diend at the same time.
Taken to an absolutely appalling level in episode 12, where Kivaara has CGI in one scene and then instantly shifts back to toy-on-a-string for her next appearance in the very same episode.
Tear Jerker: the deaths of Kuuga!Yashiro and AU!Hibiki. Some could probably counted as well.
They Wasted a Perfectly Good Plot: The whole "Orphenochs can't go to school" aspect was just mentioned by one or two lines and that is it. Even after Faiz is revealed to be one, it doesn't get mentioned again.
More than likely, Faiz's plot wasn't expanded because it was very similar to Kiva's, so rather than focusing on the Fantastic Racism, they just used the arc to introduce Kaitou.
What an Idiot: No, Fourteen, forcing your most loyal servant watch his brother get brainwashed while telling him you've been lying to him the entire time about his education program WILL HAVE NO CONSEQUENCES WHATSOEVER.
In the world of Black RX, Apollo Geist claims that Tsukasa is trapped in the parallel world of Black forever. As he is saying this, meanwhile in that other area, Schwarian leaves a portal to the world of Black RX open while fighting Decade.