Badass Army: We get to see not just the Riders in the series, but every main Rider from the original to Decade teamed up, and its every bit as awesome as it sounds!
Badass Normal: Riderman, with Gackt specifically saying in interviews that he chose to play the character because, unlike other Riders who are superhuman, Joji is just an ordinary guy...with a Swiss Army WeaponArm Cannon, but still.
He really gets to be the Badass Normal in the video for The Next Decade.
Brick Joke: When Daiki first appears, he reveals to have stolen Amazon's Gigi bracelet again. The end of the movie shows him giving it back.
The Cameo: Toshiki Kashu appears as Shoichi Tsugami at the end of the movie along with Tetsuo Kurata, having previously played the AR Black and Black RX in the show, as the original Kotaro Minami (RX).
Disney Death: Eijiro Hikari, revealed to be Dr. Shinigami and destroyed by three Final Attack Rides, appears back to his normal self in the final scene of the movie.
Early-Bird Cameo: Kamen Rider Double first appears here, taking on Shadow Moon. As with Decade's reduced episode count, it would only be a few weeks after this movie premiered that Decade would finish and Double would start, thus every summer movie from now on features the early-bird cameo of the new Rider coming to help out the current (soon to depart) rider.
Enforced Method Acting: Gets a nod in the shorts made for All Riders vs Daishocker, where Tsukasa is teaching a man how to transform into Ryuki; he has trouble slotting the Advent Deck into the V-Buckle, so Tsukasa tells him "Don't worry, let the cut take care of it." This is followed by another attempt, with a Smash Cut interrupting the motion and showing the deck being slid perfectly into place.
Fake Shemp: Most Heisei Riders just grunt or don't make any sounds at all. That's because, according to some extra material, like character guides and even the official site, they're supposed to be the original Riders, but there was only one returning actor out of the 8 previous Heisei Riders there (not including Kuuga, who was Onodera, not Godai).
The same thing happens with Showa Riders, only one actor out of the 15 Showa Riders came back so the others were voiced by replacement voice actors, most notably Tetsu Inada as Rider 1 and Tomokazu Seki as Amazon.
Foreshadowing: An inhumanly subtle piece of foreshadowing crops up when the Riders take their left at the end of the movie. Momotaros tells Tsukasa not to summon him again because he's super busy. He's referring to his constant Wolverine Publicity as described on that page the new trio of Cho Den-O movies that are in production. Yet Momo's kind enough to betray his words just once and save Tsukasa's ass from Super Shocker's army in Movie War 2010. (Mind you, that movie was released before the Cho Den-O trilogy was even announced.)
Heel–Face Revolving Door: When Tsukasa regains his memories he resumes his role as the evil leader of Great Shocker...briefly, as he is usurped by Shadow Moon and his sister. Witnessing his friends leave him for betraying them, and getting an inspirational speech about being a Kamen Rider from Riderman put him back on the track to destroy Great Shocker once and for all.
Homage: Tsukasa as the Great Leader of Dai-Shocker resembles Kamen Rider Spirits' Big Bad, Judo. Which makes sense as both are personifications of the Great Leader and have similar respective powers to transform into Riders of their era, Decade being able to transform into the nine main Hesei Kamen Riders (Kuuga to Kiva), while Judo is able to transform into the eight main Showa era Kamen Riders (Kamen Rider-1 to Super-1) as well as a golden version of ZX. Judo doesn't take Riderman's form as he is considered a secondary Rider as well as more of a Badass Normal than the others.
Legion of Doom: Dai-Shocker, an organization seeking to unite all of the evil organizations and monster races in the franchise's entire history together to defeat the Riders and conquer the multiverse.
Mad Scientist: Joji Yuki, who in this reality made both the Decadriver and Diendriver.
To clarify, even J's colossal size isn't enough to face King Dark alone, so Diend uses Final Form Ride on Decade to give J Decade's power, which destroys King Dark.
Painful Transformation: Momotaros says that the Final Form Ride transformation hurts after Decade uses it. Faiz, Blade, and Kiva don't say anything themselves...probably because A. those three riders lack proper voice actors in the movie and B. a gun, sword, and bow aren't the most talkative of objects.
Shout-Out: When all the Riders perform a Rider Kick, you can catch RX and Ryuki appear together in the same shot performing the kick - the two Rider series at the time that became localized in America.
Taking the Bullet: As Decade struggles with Super-1V3 takes the opportunity to take him down with the V3 Screw Kick, only for Yuusuke to push Tsukasa out of the way and take the kick himself, eliminating from the Rider tournament.
Tomato in the Mirror: It is revealed that Decade is the leader of Great Shocker prior to the beginning of the show, but before he could begin his grand plan of conquest, he suffered from amnesia and then became a hero. Though he had no qualms about resuming his leadership and betraying his friends. At first.
What Could Have Been: The producers said they wanted to make a modernized Riderman costume for Gackt to wear in All Riders, a la the suits for Riders 1, 2, and V3 seen in Kamen Rider: The First and Kamen Rider: The Next, but weren't able to complete it in time for filming, which is why he appears only as an untransformed Joji Yuki while Riderman is portrayed by a stuntman during the ending carnage.
The Worf Effect: The big battle has various examples of this trope if one considers the identities of the various minor Dai-Shocker members.
Rider 1 defeats Ganikomoru, a Crab/Bat monster that originally could match Rider 1, with two punches.
Skyrider defeats the Queen Ant Lord from Agito, who originally only was defeated by Agito's ultimate form, with two attacks too. Although, to be fair, Skyrider's own "base" form is an ultimate form of sorts. However, in a later scene, Agito goes on to beat up the same Queen Ant Lord in his base form too.
Kabuto single handily beat an Orphenoch, a mole Imagin, a Ganikomoru and a Shiomaneking (who originally needed to be defeated by Rider 1 and 2 fighting together) without even using Clock Up.
Decade throws the Arch Orphenoch, Faiz's final enemy who originally matched 3 Riders at once, one with a Super Mode, to the ground with a single sword slash. He does the same to the Bat Fangire, Kiva's final enemy.
Shadow Moon, easily the Big Bad of Kamen Rider Black, gets this both straight and inverted in All Riders Vs. Dai-Shocker. No one's questioning his power, but he was beating Decade and Rising Ultimate Kuuga, two Riders each with the potential to destroy the planet, at the same time without breaking a sweat. And then he was inexplicably whomped by Kamen Rider Double (who seems to be "suffering" from a major case of Advertising Power-Up) before receiving an epic Rider Kick from all other 26 Riders.
Which was immediately followed by the rise of the giant King Dark, who effortlessly defeats the puny Riders by making a shitload of explosions and beats down Kamen Rider J in two punches. Then Diend decided to Make His Decade Grow and behold, King Dark becomes a nuke by eating two punches and a Rider Kick.
Zerg Rush: The first act of Dai-Shocker's army upon the entrance of all Riders is to pelt them with a barrage of rocket-propelled Shocker Mooks.
Which turns into a Chekhov's Gun of sorts. Three words. ALL. RIDER. KICK.