Adaptational Badass: The Great Leader's original incarnation didn't put up much of a fight. This one takes all of the Riders to finally put down.
Adaptational Heroism: When the the secondary Riders show up for the All Rider Break, some of them are from Ryuki, which means a lot of them aren't the "team up and take on an Evil Overlord" type. We're lookin' at you,TakeshiAsakura (though by no means only at you.)
Bad Future: Ankh unwittingly causes a Butterfly Effect when he gets jostled hard enough to cough up a Cell Medal while in the past with New Den-O. This Cell Medal is picked up by a Shocker goon, who then takes it to General Black. As it turns out, they had been experimenting with a reverse-engineered Core Medal. The resulting Greeed ended up defeating the Double Riders, causing a future where Shocker and other such evil organizations control the world.
Big Bad: Only the Great Leader of Shocker himself.
Chekhov's Gag: at the beginning of the film, OOO fights three Imagin, believing them to be Yummy even though they don't bleed Cell Medals, but Ankh arrives and tells him they're not Yummy at all. Yet Ankh was later able to procure a Core Medal from another Imagin: Momotaros.
Continuity Porn: But of course. The most obscure reference is either Snack Amigo (Chiyoko's has turned into one; it was the hangout in the original series for about five episodes or so.) or Mitsuru and Naoki (names not chosen by accident; they're two kids who became Those Two Guys in the last arc of the original series; apparently they were from the future all along?) Also, Nokko is named for three girls with -okko names. And of course, every monster was a Monster of the Week from a past series, some every bit as old as Snack Amigo.
Demoted to Extra: Decade, despite having the specific role of Planar Champion, his appearance here has him as Fake Shemp in a quick cameo like many of the past Riders. Also, there is no hint of Ryotaro and Hana, the main human characters of the original Den-O series.
Fake Shemp: Inevitable, given the number of past heroes and villains going back to 1971. Aside from the main stars, almost all of the Kamen Riders appear in-costume only and voiced by replacement voice actors. It also affects many of the villains, along with the other four "allies of justice", though Hiroshi Miyauchi returns to voice Zubat as well as V3, and Tomokazu Seki, who previously voiced Kikaider in Android Kikaider: The Animation, voices the tokusatsu incarnation of the character here. Really, it's surprising that so many were the original actors.
For Want of a Nail: The whole reason that the Bad Future exists is because Ankh's greed in trying to get the Core Medals caused him to leave, causing Eiji to go after him and get into a scuffle. One of his Cell Medals dropped out during the scuffle and escalated to the main plot of the movie.
Freeze-Frame Bonus: during the All-Rider Break, it can be picked out that not every Rider is riding a motorcycle. Agito's Tornador is on its Hover Board form, what looks like Double's RevolGarry bringing up the rear, among others.
The Last of These Is Not Like the Others: the first promotional trailer features the DenLiner going past images of the headlining Riders in reverse chronological order (from OOO to Ichigo), that ends with the Shocker mooks.
Legion of Doom: Not Dai-Shocker, or Super Shocker. Instead its the original Shocker, only this time its ambitions to take over the world have been more fruitful thanks to the Shocker Greeed. By 2011, their ranks consist of villains and monsters from across Kamen Rider history. Thankfully, the respective riders they fought against appear to take them on once more.
Mascot Mook: The Shocker Soldiers become this in the advertising and net movies, to the point where it's downright weird to see them trying to kill people in the actual film.
One-Winged Angel: The Great Leader transforms into the Rock Great Leader after OOO defeats his normal form.
Plot Hole: A lot of plot elements are either pulled out of or disappear into nowhere, to the point where thinking about them too hard will probably make your head throb.
The Slow Path: one of the two Tagalong Kidsgets left behind in the past. He would grow up to become the Shocker scientist who removed the Double Riders' brainwashing, and, shockingly, his best friend's father.
Temporal Paradox: Naoki and Mitsuru are best friends in the contemporary era, but when Naoki gets left behind in 1971, he grows up to become Mitsuru's father, which wouldn't have been possible if they weren't friends in the first place.